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<li><a class="reference internal" href="#">4. Execution model</a><ul>
<li><a class="reference internal" href="#structure-of-a-program">4.1. Structure of a program</a></li>
<li><a class="reference internal" href="#naming-and-binding">4.2. Naming and binding</a><ul>
<li><a class="reference internal" href="#binding-of-names">4.2.1. Binding of names</a></li>
<li><a class="reference internal" href="#resolution-of-names">4.2.2. Resolution of names</a></li>
<li><a class="reference internal" href="#annotation-scopes">4.2.3. Annotation scopes</a></li>
<li><a class="reference internal" href="#lazy-evaluation">4.2.4. Lazy evaluation</a></li>
<li><a class="reference internal" href="#builtins-and-restricted-execution">4.2.5. Builtins and restricted execution</a></li>
<li><a class="reference internal" href="#interaction-with-dynamic-features">4.2.6. Interaction with dynamic features</a></li>
</ul>
</li>
<li><a class="reference internal" href="#exceptions">4.3. Exceptions</a></li>
<li><a class="reference internal" href="#runtime-components">4.4. Runtime Components</a><ul>
<li><a class="reference internal" href="#general-computing-model">4.4.1. General Computing Model</a></li>
<li><a class="reference internal" href="#python-runtime-model">4.4.2. Python Runtime Model</a></li>
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  <section id="execution-model">
<span id="execmodel"></span><h1><span class="section-number">4. </span>Execution model<a class="headerlink" href="#execution-model" title="Link to this heading">¶</a></h1>
<section id="structure-of-a-program">
<span id="prog-structure"></span><span id="index-0"></span><h2><span class="section-number">4.1. </span>Structure of a program<a class="headerlink" href="#structure-of-a-program" title="Link to this heading">¶</a></h2>
<p id="index-1">A Python program is constructed from code blocks.
A <em class="dfn">block</em> is a piece of Python program text that is executed as a unit.
The following are blocks: a module, a function body, and a class definition.
Each command typed interactively is a block.  A script file (a file given as
standard input to the interpreter or specified as a command line argument to the
interpreter) is a code block.  A script command (a command specified on the
interpreter command line with the <a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> option) is a code block.
A module run as a top level script (as module <code class="docutils literal notranslate"><span class="pre">__main__</span></code>) from the command
line using a <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate"><span class="pre">-m</span></code></a> argument is also a code block. The string
argument passed to the built-in functions <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> and <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> is a
code block.</p>
<p id="index-2">A code block is executed in an <em class="dfn">execution frame</em>.  A frame contains some
administrative information (used for debugging) and determines where and how
execution continues after the code block’s execution has completed.</p>
</section>
<section id="naming-and-binding">
<span id="naming"></span><h2><span class="section-number">4.2. </span>Naming and binding<a class="headerlink" href="#naming-and-binding" title="Link to this heading">¶</a></h2>
<section id="binding-of-names">
<span id="bind-names"></span><span id="index-3"></span><h3><span class="section-number">4.2.1. </span>Binding of names<a class="headerlink" href="#binding-of-names" title="Link to this heading">¶</a></h3>
<p id="index-4"><em class="dfn">Names</em> refer to objects.  Names are introduced by name binding operations.</p>
<p id="index-5">The following constructs bind names:</p>
<ul class="simple">
<li><p>formal parameters to functions,</p></li>
<li><p>class definitions,</p></li>
<li><p>function definitions,</p></li>
<li><p>assignment expressions,</p></li>
<li><p><a class="reference internal" href="simple_stmts.html#assignment"><span class="std std-ref">targets</span></a> that are identifiers if occurring in
an assignment:</p>
<ul>
<li><p><a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loop header,</p></li>
<li><p>after <code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code> in a <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement, <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>
clause, <a class="reference internal" href="compound_stmts.html#except-star"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except*</span></code></a> clause, or in the as-pattern in structural pattern matching,</p></li>
<li><p>in a capture pattern in structural pattern matching</p></li>
</ul>
</li>
<li><p><a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statements.</p></li>
<li><p><a class="reference internal" href="simple_stmts.html#type"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code></a> statements.</p></li>
<li><p><a class="reference internal" href="compound_stmts.html#type-params"><span class="std std-ref">type parameter lists</span></a>.</p></li>
</ul>
<p>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code> statement of the form <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span> <span class="pre">*</span></code> binds all
names defined in the imported module, except those beginning with an underscore.
This form may only be used at the module level.</p>
<p>A target occurring in a <a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement is also considered bound for
this purpose (though the actual semantics are to unbind the name).</p>
<p>Each assignment or import statement occurs within a block defined by a class or
function definition or at the module level (the top-level code block).</p>
<p id="index-6">If a name is bound in a block, it is a local variable of that block, unless
declared as <a class="reference internal" href="simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> or <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a>.  If a name is bound at
the module level, it is a global variable.  (The variables of the module code
block are local and global.)  If a variable is used in a code block but not
defined there, it is a <a class="reference internal" href="../glossary.html#term-free-variable"><span class="xref std std-term">free variable</span></a>.</p>
<p>Each occurrence of a name in the program text refers to the <em class="dfn">binding</em> of
that name established by the following name resolution rules.</p>
</section>
<section id="resolution-of-names">
<span id="resolve-names"></span><h3><span class="section-number">4.2.2. </span>Resolution of names<a class="headerlink" href="#resolution-of-names" title="Link to this heading">¶</a></h3>
<p id="index-7">A <em class="dfn">scope</em> defines the visibility of a name within a block.  If a local
variable is defined in a block, its scope includes that block.  If the
definition occurs in a function block, the scope extends to any blocks contained
within the defining one, unless a contained block introduces a different binding
for the name.</p>
<p id="index-8">When a name is used in a code block, it is resolved using the nearest enclosing
scope.  The set of all such scopes visible to a code block is called the block’s
<em class="dfn">environment</em>.</p>
<p id="index-9">When a name is not found at all, a <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a> exception is raised.
If the current scope is a function scope, and the name refers to a local
variable that has not yet been bound to a value at the point where the name is
used, an <a class="reference internal" href="../library/exceptions.html#UnboundLocalError" title="UnboundLocalError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">UnboundLocalError</span></code></a> exception is raised.
<code class="xref py py-exc docutils literal notranslate"><span class="pre">UnboundLocalError</span></code> is a subclass of <code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code>.</p>
<p>If a name binding operation occurs anywhere within a code block, all uses of the
name within the block are treated as references to the current block.  This can
lead to errors when a name is used within a block before it is bound.  This rule
is subtle.  Python lacks declarations and allows name binding operations to
occur anywhere within a code block.  The local variables of a code block can be
determined by scanning the entire text of the block for name binding operations.
See <a class="reference internal" href="../faq/programming.html#faq-unboundlocalerror"><span class="std std-ref">the FAQ entry on UnboundLocalError</span></a>
for examples.</p>
<p>If the <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement occurs within a block, all uses of the names
specified in the statement refer to the bindings of those names in the top-level
namespace.  Names are resolved in the top-level namespace by searching the
global namespace, i.e. the namespace of the module containing the code block,
and the builtins namespace, the namespace of the module <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a>.  The
global namespace is searched first.  If the names are not found there, the
builtins namespace is searched next. If the names are also not found in the
builtins namespace, new variables are created in the global namespace.
The global statement must precede all uses of the listed names.</p>
<p>The <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement has the same scope as a name binding operation
in the same block.  If the nearest enclosing scope for a free variable contains
a global statement, the free variable is treated as a global.</p>
<p>The <a class="reference internal" href="simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> statement causes corresponding names to refer
to previously bound variables in the nearest enclosing function scope.
<a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> is raised at compile time if the given name does not
exist in any enclosing function scope. <a class="reference internal" href="compound_stmts.html#type-params"><span class="std std-ref">Type parameters</span></a>
cannot be rebound with the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code> statement.</p>
<p id="index-10">The namespace for a module is automatically created the first time a module is
imported.  The main module for a script is always called <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a>.</p>
<p>Class definition blocks and arguments to <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> and <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> are
special in the context of name resolution.
A class definition is an executable statement that may use and define names.
These references follow the normal rules for name resolution with an exception
that unbound local variables are looked up in the global namespace.
The namespace of the class definition becomes the attribute dictionary of
the class. The scope of names defined in a class block is limited to the
class block; it does not extend to the code blocks of methods. This includes
comprehensions and generator expressions, but it does not include
<a class="reference internal" href="#annotation-scopes"><span class="std std-ref">annotation scopes</span></a>,
which have access to their enclosing class scopes.
This means that the following will fail:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">A</span><span class="p">:</span>
    <span class="n">a</span> <span class="o">=</span> <span class="mi">42</span>
    <span class="n">b</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">a</span> <span class="o">+</span> <span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>
</pre></div>
</div>
<p>However, the following will succeed:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">A</span><span class="p">:</span>
    <span class="nb">type</span> <span class="n">Alias</span> <span class="o">=</span> <span class="n">Nested</span>
    <span class="k">class</span><span class="w"> </span><span class="nc">Nested</span><span class="p">:</span> <span class="k">pass</span>

<span class="nb">print</span><span class="p">(</span><span class="n">A</span><span class="o">.</span><span class="n">Alias</span><span class="o">.</span><span class="n">__value__</span><span class="p">)</span>  <span class="c1"># &lt;type &#39;A.Nested&#39;&gt;</span>
</pre></div>
</div>
</section>
<section id="annotation-scopes">
<span id="id1"></span><h3><span class="section-number">4.2.3. </span>Annotation scopes<a class="headerlink" href="#annotation-scopes" title="Link to this heading">¶</a></h3>
<p><a class="reference internal" href="../glossary.html#term-annotation"><span class="xref std std-term">Annotations</span></a>, <a class="reference internal" href="compound_stmts.html#type-params"><span class="std std-ref">type parameter lists</span></a>
and <a class="reference internal" href="simple_stmts.html#type"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code></a> statements
introduce <em>annotation scopes</em>, which behave mostly like function scopes,
but with some exceptions discussed below.</p>
<p>Annotation scopes are used in the following contexts:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../glossary.html#term-function-annotation"><span class="xref std std-term">Function annotations</span></a>.</p></li>
<li><p><a class="reference internal" href="../glossary.html#term-variable-annotation"><span class="xref std std-term">Variable annotations</span></a>.</p></li>
<li><p>Type parameter lists for <a class="reference internal" href="compound_stmts.html#generic-type-aliases"><span class="std std-ref">generic type aliases</span></a>.</p></li>
<li><p>Type parameter lists for <a class="reference internal" href="compound_stmts.html#generic-functions"><span class="std std-ref">generic functions</span></a>.
A generic function’s annotations are
executed within the annotation scope, but its defaults and decorators are not.</p></li>
<li><p>Type parameter lists for <a class="reference internal" href="compound_stmts.html#generic-classes"><span class="std std-ref">generic classes</span></a>.
A generic class’s base classes and
keyword arguments are executed within the annotation scope, but its decorators are not.</p></li>
<li><p>The bounds, constraints, and default values for type parameters
(<a class="reference internal" href="#lazy-evaluation"><span class="std std-ref">lazily evaluated</span></a>).</p></li>
<li><p>The value of type aliases (<a class="reference internal" href="#lazy-evaluation"><span class="std std-ref">lazily evaluated</span></a>).</p></li>
</ul>
<p>Annotation scopes differ from function scopes in the following ways:</p>
<ul class="simple">
<li><p>Annotation scopes have access to their enclosing class namespace.
If an annotation scope is immediately within a class scope, or within another
annotation scope that is immediately within a class scope, the code in the
annotation scope can use names defined in the class scope as if it were
executed directly within the class body. This contrasts with regular
functions defined within classes, which cannot access names defined in the class scope.</p></li>
<li><p>Expressions in annotation scopes cannot contain <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a>, <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code>,
<a class="reference internal" href="expressions.html#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a>, or <a class="reference internal" href="expressions.html#grammar-token-python-grammar-assignment_expression"><code class="xref std std-token docutils literal notranslate"><span class="pre">:=</span></code></a>
expressions. (These expressions are allowed in other scopes contained within the
annotation scope.)</p></li>
<li><p>Names defined in annotation scopes cannot be rebound with <a class="reference internal" href="simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a>
statements in inner scopes. This includes only type parameters, as no other
syntactic elements that can appear within annotation scopes can introduce new names.</p></li>
<li><p>While annotation scopes have an internal name, that name is not reflected in the
<a class="reference internal" href="../glossary.html#term-qualified-name"><span class="xref std std-term">qualified name</span></a> of objects defined within the scope.
Instead, the <a class="reference internal" href="../library/stdtypes.html#definition.__qualname__" title="definition.__qualname__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__qualname__</span></code></a>
of such objects is as if the object were defined in the enclosing scope.</p></li>
</ul>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.12: </span>Annotation scopes were introduced in Python 3.12 as part of <span class="target" id="index-11"></span><a class="pep reference external" href="https://peps.python.org/pep-0695/"><strong>PEP 695</strong></a>.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.13: </span>Annotation scopes are also used for type parameter defaults, as
introduced by <span class="target" id="index-12"></span><a class="pep reference external" href="https://peps.python.org/pep-0696/"><strong>PEP 696</strong></a>.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.14: </span>Annotation scopes are now also used for annotations, as specified in
<span class="target" id="index-13"></span><a class="pep reference external" href="https://peps.python.org/pep-0649/"><strong>PEP 649</strong></a> and <span class="target" id="index-14"></span><a class="pep reference external" href="https://peps.python.org/pep-0749/"><strong>PEP 749</strong></a>.</p>
</div>
</section>
<section id="lazy-evaluation">
<span id="id2"></span><h3><span class="section-number">4.2.4. </span>Lazy evaluation<a class="headerlink" href="#lazy-evaluation" title="Link to this heading">¶</a></h3>
<p>Most annotation scopes are <em>lazily evaluated</em>. This includes annotations,
the values of type aliases created through the <a class="reference internal" href="simple_stmts.html#type"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code></a> statement, and
the bounds, constraints, and default values of type
variables created through the <a class="reference internal" href="compound_stmts.html#type-params"><span class="std std-ref">type parameter syntax</span></a>.
This means that they are not evaluated when the type alias or type variable is
created, or when the object carrying annotations is created. Instead, they
are only evaluated when necessary, for example when the <code class="docutils literal notranslate"><span class="pre">__value__</span></code>
attribute on a type alias is accessed.</p>
<p>Example:</p>
<div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="nb">type</span> <span class="n">Alias</span> <span class="o">=</span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">Alias</span><span class="o">.</span><span class="n">__value__</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="w">  </span><span class="c">...</span>
<span class="gr">ZeroDivisionError</span>: <span class="n">division by zero</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">def</span><span class="w"> </span><span class="nf">func</span><span class="p">[</span><span class="n">T</span><span class="p">:</span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span><span class="p">]():</span> <span class="k">pass</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">T</span> <span class="o">=</span> <span class="n">func</span><span class="o">.</span><span class="n">__type_params__</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">T</span><span class="o">.</span><span class="n">__bound__</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="w">  </span><span class="c">...</span>
<span class="gr">ZeroDivisionError</span>: <span class="n">division by zero</span>
</pre></div>
</div>
<p>Here the exception is raised only when the <code class="docutils literal notranslate"><span class="pre">__value__</span></code> attribute
of the type alias or the <code class="docutils literal notranslate"><span class="pre">__bound__</span></code> attribute of the type variable
is accessed.</p>
<p>This behavior is primarily useful for references to types that have not
yet been defined when the type alias or type variable is created. For example,
lazy evaluation enables creation of mutually recursive type aliases:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">typing</span><span class="w"> </span><span class="kn">import</span> <span class="n">Literal</span>

<span class="nb">type</span> <span class="n">SimpleExpr</span> <span class="o">=</span> <span class="nb">int</span> <span class="o">|</span> <span class="n">Parenthesized</span>
<span class="nb">type</span> <span class="n">Parenthesized</span> <span class="o">=</span> <span class="nb">tuple</span><span class="p">[</span><span class="n">Literal</span><span class="p">[</span><span class="s2">&quot;(&quot;</span><span class="p">],</span> <span class="n">Expr</span><span class="p">,</span> <span class="n">Literal</span><span class="p">[</span><span class="s2">&quot;)&quot;</span><span class="p">]]</span>
<span class="nb">type</span> <span class="n">Expr</span> <span class="o">=</span> <span class="n">SimpleExpr</span> <span class="o">|</span> <span class="nb">tuple</span><span class="p">[</span><span class="n">SimpleExpr</span><span class="p">,</span> <span class="n">Literal</span><span class="p">[</span><span class="s2">&quot;+&quot;</span><span class="p">,</span> <span class="s2">&quot;-&quot;</span><span class="p">],</span> <span class="n">Expr</span><span class="p">]</span>
</pre></div>
</div>
<p>Lazily evaluated values are evaluated in <a class="reference internal" href="#annotation-scopes"><span class="std std-ref">annotation scope</span></a>,
which means that names that appear inside the lazily evaluated value are looked up
as if they were used in the immediately enclosing scope.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.12.</span></p>
</div>
</section>
<section id="builtins-and-restricted-execution">
<span id="restrict-exec"></span><h3><span class="section-number">4.2.5. </span>Builtins and restricted execution<a class="headerlink" href="#builtins-and-restricted-execution" title="Link to this heading">¶</a></h3>
<div class="impl-detail compound" id="index-15">
<p><strong>CPython implementation detail:</strong> Users should not touch <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code>; it is strictly an implementation
detail.  Users wanting to override values in the builtins namespace should
<a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> the <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a> module and modify its
attributes appropriately.</p>
</div>
<p>The builtins namespace associated with the execution of a code block
is actually found by looking up the name <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code> in its
global namespace; this should be a dictionary or a module (in the
latter case the module’s dictionary is used).  By default, when in the
<a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module, <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code> is the built-in module
<a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a>; when in any other module, <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code> is an
alias for the dictionary of the <code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code> module itself.</p>
</section>
<section id="interaction-with-dynamic-features">
<span id="dynamic-features"></span><h3><span class="section-number">4.2.6. </span>Interaction with dynamic features<a class="headerlink" href="#interaction-with-dynamic-features" title="Link to this heading">¶</a></h3>
<p>Name resolution of free variables occurs at runtime, not at compile time.
This means that the following code will print 42:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">=</span> <span class="mi">10</span>
<span class="k">def</span><span class="w"> </span><span class="nf">f</span><span class="p">():</span>
    <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="n">i</span> <span class="o">=</span> <span class="mi">42</span>
<span class="n">f</span><span class="p">()</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> and <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> functions do not have access to the full
environment for resolving names.  Names may be resolved in the local and global
namespaces of the caller.  Free variables are not resolved in the nearest
enclosing namespace, but in the global namespace.  <a class="footnote-reference brackets" href="#id5" id="id3" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a> The <code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code> and
<code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code> functions have optional arguments to override the global and local
namespace.  If only one namespace is specified, it is used for both.</p>
</section>
</section>
<section id="exceptions">
<span id="id4"></span><h2><span class="section-number">4.3. </span>Exceptions<a class="headerlink" href="#exceptions" title="Link to this heading">¶</a></h2>
<p id="index-17"><span id="index-16"></span>Exceptions are a means of breaking out of the normal flow of control of a code
block in order to handle errors or other exceptional conditions.  An exception
is <em>raised</em> at the point where the error is detected; it may be <em>handled</em> by the
surrounding code block or by any code block that directly or indirectly invoked
the code block where the error occurred.</p>
<p>The Python interpreter raises an exception when it detects a run-time error
(such as division by zero).  A Python program can also explicitly raise an
exception with the <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement. Exception handlers are specified
with the <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> … <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> statement.  The <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a>
clause of such a statement can be used to specify cleanup code which does not
handle the exception, but is executed whether an exception occurred or not in
the preceding code.</p>
<p id="index-18">Python uses the “termination” model of error handling: an exception handler can
find out what happened and continue execution at an outer level, but it cannot
repair the cause of the error and retry the failing operation (except by
re-entering the offending piece of code from the top).</p>
<p id="index-19">When an exception is not handled at all, the interpreter terminates execution of
the program, or returns to its interactive main loop.  In either case, it prints
a stack traceback, except when the exception is <a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a>.</p>
<p>Exceptions are identified by class instances.  The <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clause is
selected depending on the class of the instance: it must reference the class of
the instance or a <a class="reference internal" href="../glossary.html#term-abstract-base-class"><span class="xref std std-term">non-virtual base class</span></a> thereof.
The instance can be received by the handler and can carry additional information
about the exceptional condition.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Exception messages are not part of the Python API.  Their contents may change
from one version of Python to the next without warning and should not be
relied on by code which will run under multiple versions of the interpreter.</p>
</div>
<p>See also the description of the <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement in section <a class="reference internal" href="compound_stmts.html#try"><span class="std std-ref">The try statement</span></a>
and <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement in section <a class="reference internal" href="simple_stmts.html#raise"><span class="std std-ref">The raise statement</span></a>.</p>
</section>
<section id="runtime-components">
<span id="execcomponents"></span><h2><span class="section-number">4.4. </span>Runtime Components<a class="headerlink" href="#runtime-components" title="Link to this heading">¶</a></h2>
<section id="general-computing-model">
<h3><span class="section-number">4.4.1. </span>General Computing Model<a class="headerlink" href="#general-computing-model" title="Link to this heading">¶</a></h3>
<p>Python’s execution model does not operate in a vacuum.  It runs on
a host machine and through that host’s runtime environment, including
its operating system (OS), if there is one.  When a program runs,
the conceptual layers of how it runs on the host look something
like this:</p>
<blockquote>
<div><div class="line-block">
<div class="line"><strong>host machine</strong></div>
<div class="line-block">
<div class="line"><strong>process</strong> (global resources)</div>
<div class="line-block">
<div class="line"><strong>thread</strong> (runs machine code)</div>
</div>
</div>
</div>
</div></blockquote>
<p>Each process represents a program running on the host.  Think of each
process itself as the data part of its program.  Think of the process’
threads as the execution part of the program.  This distinction will
be important to understand the conceptual Python runtime.</p>
<p>The process, as the data part, is the execution context in which the
program runs.  It mostly consists of the set of resources assigned to
the program by the host, including memory, signals, file handles,
sockets, and environment variables.</p>
<p>Processes are isolated and independent from one another.  (The same
is true for hosts.)  The host manages the process’ access to its
assigned resources, in addition to coordinating between processes.</p>
<p>Each thread represents the actual execution of the program’s machine
code, running relative to the resources assigned to the program’s
process.  It’s strictly up to the host how and when that execution
takes place.</p>
<p>From the point of view of Python, a program always starts with exactly
one thread.  However, the program may grow to run in multiple
simultaneous threads.  Not all hosts support multiple threads per
process, but most do.  Unlike processes, threads in a process are not
isolated and independent from one another.  Specifically, all threads
in a process share all of the process’ resources.</p>
<p>The fundamental point of threads is that each one does <em>run</em>
independently, at the same time as the others.  That may be only
conceptually at the same time (“concurrently”) or physically
(“in parallel”).  Either way, the threads effectively run
at a non-synchronized rate.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>That non-synchronized rate means none of the process’ memory is
guaranteed to stay consistent for the code running in any given
thread.  Thus multi-threaded programs must take care to coordinate
access to intentionally shared resources.  Likewise, they must take
care to be absolutely diligent about not accessing any <em>other</em>
resources in multiple threads; otherwise two threads running at the
same time might accidentally interfere with each other’s use of some
shared data.  All this is true for both Python programs and the
Python runtime.</p>
<p>The cost of this broad, unstructured requirement is the tradeoff for
the kind of raw concurrency that threads provide.  The alternative
to the required discipline generally means dealing with
non-deterministic bugs and data corruption.</p>
</div>
</section>
<section id="python-runtime-model">
<h3><span class="section-number">4.4.2. </span>Python Runtime Model<a class="headerlink" href="#python-runtime-model" title="Link to this heading">¶</a></h3>
<p>The same conceptual layers apply to each Python program, with some
extra data layers specific to Python:</p>
<blockquote>
<div><div class="line-block">
<div class="line"><strong>host machine</strong></div>
<div class="line-block">
<div class="line"><strong>process</strong> (global resources)</div>
<div class="line-block">
<div class="line">Python global runtime (<em>state</em>)</div>
<div class="line-block">
<div class="line">Python interpreter (<em>state</em>)</div>
<div class="line-block">
<div class="line"><strong>thread</strong> (runs Python bytecode and “C-API”)</div>
<div class="line-block">
<div class="line">Python thread <em>state</em></div>
</div>
</div>
</div>
</div>
</div>
</div>
</div></blockquote>
<p>At the conceptual level: when a Python program starts, it looks exactly
like that diagram, with one of each.  The runtime may grow to include
multiple interpreters, and each interpreter may grow to include
multiple thread states.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>A Python implementation won’t necessarily implement the runtime
layers distinctly or even concretely.  The only exception is places
where distinct layers are directly specified or exposed to users,
like through the <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.</p>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The initial interpreter is typically called the “main” interpreter.
Some Python implementations, like CPython, assign special roles
to the main interpreter.</p>
<p>Likewise, the host thread where the runtime was initialized is known
as the “main” thread.  It may be different from the process’ initial
thread, though they are often the same.  In some cases “main thread”
may be even more specific and refer to the initial thread state.
A Python runtime might assign specific responsibilities
to the main thread, such as handling signals.</p>
</div>
<p>As a whole, the Python runtime consists of the global runtime state,
interpreters, and thread states.  The runtime ensures all that state
stays consistent over its lifetime, particularly when used with
multiple host threads.</p>
<p>The global runtime, at the conceptual level, is just a set of
interpreters.  While those interpreters are otherwise isolated and
independent from one another, they may share some data or other
resources.  The runtime is responsible for managing these global
resources safely.  The actual nature and management of these resources
is implementation-specific.  Ultimately, the external utility of the
global runtime is limited to managing interpreters.</p>
<p>In contrast, an “interpreter” is conceptually what we would normally
think of as the (full-featured) “Python runtime”.  When machine code
executing in a host thread interacts with the Python runtime, it calls
into Python in the context of a specific interpreter.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The term “interpreter” here is not the same as the “bytecode
interpreter”, which is what regularly runs in threads, executing
compiled Python code.</p>
<p>In an ideal world, “Python runtime” would refer to what we currently
call “interpreter”.  However, it’s been called “interpreter” at least
since introduced in 1997 (<a class="reference external" href="https://github.com/python/cpython/commit/a027efa5b">CPython:a027efa5b</a>).</p>
</div>
<p>Each interpreter completely encapsulates all of the non-process-global,
non-thread-specific state needed for the Python runtime to work.
Notably, the interpreter’s state persists between uses.  It includes
fundamental data like <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a>.  The runtime ensures
multiple threads using the same interpreter will safely
share it between them.</p>
<p>A Python implementation may support using multiple interpreters at the
same time in the same process.  They are independent and isolated from
one another.  For example, each interpreter has its own
<a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a>.</p>
<p>For thread-specific runtime state, each interpreter has a set of thread
states, which it manages, in the same way the global runtime contains
a set of interpreters.  It can have thread states for as many host
threads as it needs.  It may even have multiple thread states for
the same host thread, though that isn’t as common.</p>
<p>Each thread state, conceptually, has all the thread-specific runtime
data an interpreter needs to operate in one host thread.  The thread
state includes the current raised exception and the thread’s Python
call stack.  It may include other thread-specific resources.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The term “Python thread” can sometimes refer to a thread state, but
normally it means a thread created using the <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.</p>
</div>
<p>Each thread state, over its lifetime, is always tied to exactly one
interpreter and exactly one host thread.  It will only ever be used in
that thread and with that interpreter.</p>
<p>Multiple thread states may be tied to the same host thread, whether for
different interpreters or even the same interpreter.  However, for any
given host thread, only one of the thread states tied to it can be used
by the thread at a time.</p>
<p>Thread states are isolated and independent from one another and don’t
share any data, except for possibly sharing an interpreter and objects
or other resources belonging to that interpreter.</p>
<p>Once a program is running, new Python threads can be created using the
<a class="reference internal" href="../library/threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module (on platforms and Python implementations that
support threads).  Additional processes can be created using the
<a class="reference internal" href="../library/os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a>, <a class="reference internal" href="../library/subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate"><span class="pre">subprocess</span></code></a>, and <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> modules.
Interpreters can be created and used with the
<a class="reference internal" href="../library/concurrent.interpreters.html#module-concurrent.interpreters" title="concurrent.interpreters: Multiple interpreters in the same process"><code class="xref py py-mod docutils literal notranslate"><span class="pre">interpreters</span></code></a> module.  Coroutines (async) can
be run using <a class="reference internal" href="../library/asyncio.html#module-asyncio" title="asyncio: Asynchronous I/O."><code class="xref py py-mod docutils literal notranslate"><span class="pre">asyncio</span></code></a> in each interpreter, typically only
in a single thread (often the main thread).</p>
<p class="rubric">Footnotes</p>
<aside class="footnote-list brackets">
<aside class="footnote brackets" id="id5" role="doc-footnote">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id3">1</a><span class="fn-bracket">]</span></span>
<p>This limitation occurs because the code that is executed by these operations
is not available at the time the module is compiled.</p>
</aside>
</aside>
</section>
</section>
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<li><a class="reference internal" href="#">4. Execution model</a><ul>
<li><a class="reference internal" href="#structure-of-a-program">4.1. Structure of a program</a></li>
<li><a class="reference internal" href="#naming-and-binding">4.2. Naming and binding</a><ul>
<li><a class="reference internal" href="#binding-of-names">4.2.1. Binding of names</a></li>
<li><a class="reference internal" href="#resolution-of-names">4.2.2. Resolution of names</a></li>
<li><a class="reference internal" href="#annotation-scopes">4.2.3. Annotation scopes</a></li>
<li><a class="reference internal" href="#lazy-evaluation">4.2.4. Lazy evaluation</a></li>
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<li><a class="reference internal" href="#runtime-components">4.4. Runtime Components</a><ul>
<li><a class="reference internal" href="#general-computing-model">4.4.1. General Computing Model</a></li>
<li><a class="reference internal" href="#python-runtime-model">4.4.2. Python Runtime Model</a></li>
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