string.Template doesn't work with the self keyword argument

[serhiy-storchaka]

BPO	23671
Nosy	@birkenfeld, @terryjreedy, @merwok, @serhiy-storchaka
Files	string_formatting_self.patch string_formatting_self_2.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2015-03-24.20:33:12.816>
created_at = <Date 2015-03-15.08:21:34.296>
labels = ['type-bug', 'library']
title = "string.Template doesn't work with the self keyword argument"
updated_at = <Date 2015-03-27.16:13:15.216>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-03-27.16:13:15.216>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-03-24.20:33:12.816>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2015-03-15.08:21:34.296>
creator = 'serhiy.storchaka'
dependencies = []
files = ['38497', '38590']
hgrepos = []
issue_num = 23671
keywords = ['patch']
message_count = 6.0
messages = ['238132', '238631', '239177', '239183', '239412', '239414']
nosy_count = 5.0
nosy_names = ['georg.brandl', 'terry.reedy', 'eric.araujo', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23671'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5']

[serhiy-storchaka]

string.Template doesn't allow to specify the self substitute parameter as keyword argument.

>>> import string
>>> string.Template('the self is $self').substitute(self='bozo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: substitute() got multiple values for argument 'self'
>>> string.Template('the self is $self').safe_substitute(self='bozo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: safe_substitute() got multiple values for argument 'self'

The same issue is with string.Formatter.format:

>>> string.Formatter().format('the self is {self}', self='bozo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: format() got multiple values for argument 'self'

Proposed patch fixes these issues.

[serhiy-storchaka]

Updated patch addresses Paul's and Demian's comments.

Converting the format_string parameter to positional parameter can break third-party code, so this needs a deprecation period.

[python-dev]

New changeset 1c19778123a3 by Serhiy Storchaka in branch '2.7':
Issue bpo-23671: string.Template now allows to specify the "self" parameter as
https://hg.python.org/cpython/rev/1c19778123a3

New changeset 7a4b499c4dc0 by Serhiy Storchaka in branch '3.4':
Issue bpo-23671: string.Template now allows to specify the "self" parameter as
https://hg.python.org/cpython/rev/7a4b499c4dc0

New changeset 0dd263949e41 by Serhiy Storchaka in branch 'default':
Issue bpo-23671: string.Template now allows to specify the "self" parameter as
https://hg.python.org/cpython/rev/0dd263949e41

[terryjreedy]

This is a nice catch of a subtle bug and a nice fix. I verified that replacing 'self' with '*args' does not simply shift the problem from 'self' to 'args'.

>>> class C:
	def test(*args, **kwargs):
		print(args, kwargs )
	
>>> c = C()
>>> c.test(1, args=2, kwargs=3, self=4)
(<__main__.C object at 0x00000000035FE128>, 1) {'args': 2, 'kwargs': 3, 'self': 4}

While the * and ** names are, like parameter names, local names given in the signature header, they are not counted as parameter names in the code object .co_argcount or .co_kwonlyargcount attributes that are used along with co_varnames in the name-argument matching process.

[merwok]

"descriptor 'substitute' of 'Template' object needs an argument"

These error messages don’t seem very user-friendly. I think the style in the rest of the module is like "substitute method wants x y z".

[serhiy-storchaka]

It matches error messages generated by builtin unbound methods.

>>> str.format()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor 'format' of 'str' object needs an argument

It would be incorrect to say "substitute method wants x y z", because the substitute method doesn't need any arguments.

>>> string.Template('spam').substitute()
'spam'