matplotlib

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#!/usr/bin/env python3
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"""
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Generate matplotlirc for installs.
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If packagers want to change the default backend, insert a `#backend: ...` line.
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Otherwise, use the default `##backend: Agg` which has no effect even after
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decommenting, which allows _auto_backend_sentinel to be filled in at import
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time.
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"""
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import sys
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from pathlib import Path
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if len(sys.argv) != 4:
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    raise SystemExit('usage: {sys.argv[0]} <input> <output> <backend>')
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input = Path(sys.argv[1])
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output = Path(sys.argv[2])
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backend = sys.argv[3]
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template_lines = input.read_text(encoding="utf-8").splitlines(True)
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backend_line_idx, = (  # Also asserts that there is a single such line.
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    idx for idx, line in enumerate(template_lines)
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    if "#backend:" in line)
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template_lines[backend_line_idx] = (
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    f"#backend: {backend}\n" if backend not in ['', 'auto'] else "##backend: Agg\n")
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output.write_text("".join(template_lines), encoding="utf-8")
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