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Lib/string.py

cpython 3.14 @ ab2d84fe1023/Lib/string.py

Lib/string.py provides the Formatter class (the implementation behind str.format()), Template string substitution, capwords, and the character-set constants.

Map​

LinesSymbolRole
1-40Character constantsascii_letters, digits, punctuation, whitespace, printable
41-80capwordsSplit + capitalize + join
81-220Formatterformat(), vformat(), parse(), get_field(), format_field()
221-320Template$identifier / ${identifier} substitution with safe_substitute

Reading​

Formatter.vformat​

vformat calls _vformat which iterates the format string, extracting literal text and field placeholders via parse(). For each {field_name!conversion:format_spec} it calls get_field(field_name, args, kwargs) to resolve the value, applies the conversion (!r / !s / !a), and calls format_field(value, format_spec).

# CPython: Lib/string.py:162 _vformat
def _vformat(self, format_string, args, kwargs, used_args, recursion_depth, ...):
    ...
    for literal_text, field_name, format_spec, conversion in self.parse(format_string):
        ...
        obj, arg_used = self.get_field(field_name, args, kwargs)
        obj = self.convert_field(obj, conversion)
        ...
        result.append(self.format_field(obj, format_spec))
    return ''.join(result)

Template substitution​

Template uses a compiled regex to find $identifier and ${identifier} patterns. substitute raises KeyError for missing keys; safe_substitute leaves unmatched patterns unchanged.

Formatter.parse​

parse(format_string) yields (literal_text, field_name, format_spec, conversion) tuples by walking the format string character by character, handling nested {} for format specs.

gopy notes​

str.format() calls the Formatter protocol internally through the C str object. A pure- Python Formatter in module/string/ will be available for user subclassing. Template requires only string operations and re.