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.

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.

New changeset 1c19778123a3 by Serhiy Storchaka in branch '2.7':
Issue #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 #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 #23671: string.Template now allows to specify the "self" parameter as
https://hg.python.org/cpython/rev/0dd263949e41

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.

"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".

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'