ChainMap.new_child could use improvement

[vsajip]

BPO	16613
Nosy	@doerwalter, @rhettinger, @vsajip, @bitdancer
Files	new-child.diff: Code, test, and doc change for feature.

^{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/vsajip'
closed_at = <Date 2013-01-11.23:40:07.204>
created_at = <Date 2012-12-05.09:13:11.098>
labels = ['type-feature', 'library']
title = 'ChainMap.new_child could use improvement'
updated_at = <Date 2013-01-11.23:40:07.203>
user = 'https://github.com/vsajip'

bugs.python.org fields:

activity = <Date 2013-01-11.23:40:07.203>
actor = 'python-dev'
assignee = 'vinay.sajip'
closed = True
closed_date = <Date 2013-01-11.23:40:07.204>
closer = 'python-dev'
components = ['Library (Lib)']
creation = <Date 2012-12-05.09:13:11.098>
creator = 'vinay.sajip'
dependencies = []
files = ['28694']
hgrepos = ['171']
issue_num = 16613
keywords = ['patch']
message_count = 9.0
messages = ['176974', '179307', '179552', '179563', '179671', '179674', '179728', '179736', '179745']
nosy_count = 5.0
nosy_names = ['doerwalter', 'rhettinger', 'vinay.sajip', 'r.david.murray', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue16613'
versions = ['Python 3.4']

[vsajip]

ChainMap.new_child could IMO be improved through allowing an optional dict to be passed, which is used to create the child. The use case is that you sometimes need to temporarily push a new non-empty mapping in front of an existing chain. This could be achieved by changing new_child to the following, which is backwards-compatible:

    def new_child(self, d=None):
        'New ChainMap with a new dict followed by all previous maps.'
        return self.__class__(d or {}, *self.maps)

[bitdancer]

I agree that this would be useful.

[doerwalter]

I'd like to have this feature too. However the code should use

d if d is not None else {}

instead of

d or {}

For example I might want to use a subclass of dict (lowerdict) that converts all keys to lowercase. When I use an empty lowerdict in new_child(), new_child() would silently use a normal dict instead:

   class lowerdict(dict):
       def __getitem__(self, key):
           return dict.__getitem__(
               self,
               key.lower() if isinstance(key, str) else key
           )
   
   import collections
   
   cm = collections.ChainMap(lowerdict(), lowerdict())
   
   cm2 = cm.new_child(lowerdict())
   
   print(type(cm2.maps[0]))

This would print <class 'dict'>.

[vsajip]

d if d is not None else {}

Your intention makes sense, though I would prefer to write it as:

    if d is None:
        d = {}
    return self.__class__(d, *self.maps)

[rhettinger]

Can you write-up a patch with tests and a doc change?

[vsajip]

Can you write-up a patch with tests and a doc change?

Done.

[rhettinger]

Put a *versionchanged* tag in the doc entry and this is ready to apply.

[rhettinger]

Also, please change the variable name from *amap* to either *m* or *mapping*.

[python-dev]

New changeset c0ddae67f4df by Vinay Sajip in branch 'default':
Closes bpo-16613: Added optional mapping argument to ChainMap.new_child.
http://hg.python.org/cpython/rev/c0ddae67f4df