[Python-Dev] arguments policy: **kwargs.pop()

[Tres Seaver]

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On 04/10/2014 10:12 PM, Christian Tismer wrote:

> I always used the policy that arguments are never changed by a
> function, unless explicitly stated. But since I see this pattern quite
> frequently, I wanted to ask if I am right by thinking this way, or if
> the general policy is more like "arguments may be destroyed by
> default".
> 
> What do you think? Is this bad style and should be noticed somewhere, 
> or is the caller supposed to protect the arguments, or are my worries 
> useless?

The caller can't know or care that the function / method pops arguments::

    $ python
    Python 2.7.3 (default, Feb 27 2014, 19:58:35)
    [GCC 4.6.3] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> def foo(**kw):
    ...     bar = kw.pop('bar', 'BAR')
    ...     print 'bar: %s' % bar
    ...     print 'kw: %s' % kw
    ...
    >>> foo()
    bar: BAR
    kw: {}
    >>> foo(bar='baz')
    bar: baz
    kw: {}
    >>> foo(bar='baz', bam='qux')
    bar: baz
    kw: {'bam': 'qux'}
    >>> mykw = {'bar': 'baz', 'bam': 'qux'}
    >>> foo(**mykw)
    bar: baz
    kw: {'bam': 'qux'}
    >>> mykw
    {'bam': 'qux', 'bar': 'baz'}

because the caller gets its own copy of 'kw', even when called with an
existing dict.


Tres.
- -- 
===================================================================
Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
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