Feature Requests item #1706256, was opened at 2007-04-23 20:18
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Calvin Spealman (ironfroggy)
Assigned to: Nobody/Anonymous (nobody)
Summary: Give Partial the ability to skip positionals
Initial Comment:
There are some situations where you want to skip positional arguments in a use
of a partial function. In other words, you want to create a partial that
applies positional arguments out of order or without applying values to one or
more lower positional arguments. In some cases keyword arguments can be used
instead, but this has two obvious drawbacks. Firstly, it causes the caller to
rely on the name of a positional in a callee, which breaks encapsulation.
Secondly, on the case of the function being applied to being a builtin, it
fails completely, as they will not take positional arguments by name at all. I
propose a class attribute to the partial type, 'skip', which will be a
singleton to pass to a partial object signifying this skipping of positionals.
The following example demonstrates.
from functools import partial
def add(a, b):
return a + b
append_abc = partial(add, partial.skip, "abc")
assert append_abc("xyz") == "xyzabc"
Obviously this example would break if used as partial(add, b="abc") and the
maintainer of add changed the positional names to 'first' and 'second' or 'pre'
and 'post', which is perfectly reasonable. We do not need to expect the names
of our positional arguments are depended upon. It would also break when someone
gets smart and replaces the add function with operator.add, of course.
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