Bug description

string.Template compiles its substitution regex lazily and caches it on the class. Template._compile_pattern() reads the current pattern via cls.__dict__.get('pattern', _TemplatePattern). On the free-threaded build (--disable-gil), two concurrent first uses of a template can race: once one thread has compiled the pattern and stored the resulting re.Pattern back on the class, a second thread that entered _compile_pattern() reads that already-compiled re.Pattern and falls through to

re.compile(pattern, cls.flags | re.VERBOSE)

re.compile() rejects a flags argument when it is handed an already-compiled pattern, so the second thread raises

ValueError: cannot process flags argument with a compiled pattern

Only the first concurrent use of the base Template class is exposed. Subclasses compile their pattern eagerly in __init_subclass__, so they never reach _compile_pattern() concurrently on first use.

Reproduction

On a free-threaded build, starting several threads that each perform a first Template(...).substitute(...) reliably triggers the ValueError. Racing 16 threads across a fresh process on their first use of string.Template hit the ValueError in 40 of 40 processes; with the fix below applied, 0 of 40.

There is a deterministic, single-threaded form of the same bug: a subclass that supplies an already-compiled pattern raises the same ValueError at class-definition time, via __init_subclass__ -> _compile_pattern() -> re.compile(compiled, flags):

import re
from string import Template
class T(Template):
    pattern = re.compile(r'\$(?:(?P<escaped>\$)|(?P<named>[a-z]+)|\{(?P<braced>[a-z]+)\}|(?P<invalid>))')
# ValueError: cannot process flags argument with a compiled pattern

Fix

Return the already-compiled pattern from _compile_pattern() when the stored pattern is already an re.Pattern, instead of trying to recompile it. This is idempotent and lock-free (the racing threads produce equivalent compiled patterns; last writer wins). It also fixes the deterministic subclass case above.

Linked PRs

• gh-153056: Fix a data race compiling the string.Template pattern in free-threading builds #153057
• [3.15] gh-153056: Fix a data race compiling the string.Template pattern in free-threading builds (GH-153057) #153302
• [3.14] gh-153056: Fix a data race compiling the string.Template pattern in free-threading builds (GH-153057) #153303
• [3.13] gh-153056: Backport the relevant bits of (#153057) #153369
• gh-153056: Backport the relevant bits of (#153057) #153373