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bpo-38208: Simplify string.Template by using __init_subclass__(). #16256

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merged 3 commits into from Oct 21, 2019
Merged

bpo-38208: Simplify string.Template by using __init_subclass__(). #16256

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ce02c9b
bpo-38208: Simplify string.Template by using __init_subclass__().
serhiy-storchaka Sep 18, 2019
606a4da
Remove unrelated changes.
serhiy-storchaka Sep 18, 2019
68d895b
Address review comments.
serhiy-storchaka Sep 18, 2019
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46 changes: 22 additions & 24 deletions Lib/string.py
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Expand Up @@ -54,30 +54,7 @@ def capwords(s, sep=None):

_sentinel_dict = {}

class _TemplateMetaclass(type):
pattern = r"""
%(delim)s(?:
(?P<escaped>%(delim)s) | # Escape sequence of two delimiters
(?P<named>%(id)s) | # delimiter and a Python identifier
{(?P<braced>%(bid)s)} | # delimiter and a braced identifier
(?P<invalid>) # Other ill-formed delimiter exprs
)
"""

def __init__(cls, name, bases, dct):
super(_TemplateMetaclass, cls).__init__(name, bases, dct)
if 'pattern' in dct:
pattern = cls.pattern
else:
pattern = _TemplateMetaclass.pattern % {
'delim' : _re.escape(cls.delimiter),
'id' : cls.idpattern,
'bid' : cls.braceidpattern or cls.idpattern,
}
cls.pattern = _re.compile(pattern, cls.flags | _re.VERBOSE)


class Template(metaclass=_TemplateMetaclass):
class Template:
"""A string class for supporting $-substitutions."""

delimiter = '$'
Expand All @@ -89,6 +66,24 @@ class Template(metaclass=_TemplateMetaclass):
braceidpattern = None
flags = _re.IGNORECASE

def __init_subclass__(cls):
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This seems like a better approach, but if you're modernizing this then I have a few other suggestions.

super().__init_subclass__()
if 'pattern' in cls.__dict__:
pattern = cls.pattern
else:
delim = _re.escape(cls.delimiter)
id = cls.idpattern
bid = cls.braceidpattern or cls.idpattern
pattern = fr"""
{delim}(?:
(?P<escaped>{delim}) | # Escape sequence of two delimiters
(?P<named>{id}) | # delimiter and a Python identifier
{{(?P<braced>{bid})}} | # delimiter and a braced identifier
(?P<invalid>) # Other ill-formed delimiter exprs
)
"""
cls.pattern = _re.compile(pattern, cls.flags | _re.VERBOSE)

def __init__(self, template):
self.template = template

Expand Down Expand Up @@ -146,6 +141,9 @@ def convert(mo):
self.pattern)
return self.pattern.sub(convert, self.template)

# Initialize Template.pattern. __init_subclass__() is automatically called
# only for subclasses, not for the Template class itself.
Template.__init_subclass__()


########################################################################
Expand Down