[issue30689] len() and iter() of ChainMap don't work with unhashable keys

[ebw]

ebw  added the comment:

I actually have a real life use case of that problem. Using the eval() function 
of pandas need to use a custom resolver:

class Resolver(object):
def __getitem__(self, name):
if name == "test":
return 0.5
else:
raise KeyError

resolvers = (Resolver(), )
pd.eval("test", resolvers=resolvers)

But pandas store the resolvers used for eval() in a DeepChainMap and uses 
bool(len(self.resolvers)) in the property has_resolvers. Which with my custom 
Resolver raise a KeyError because __getiem__ gets call with name = 0.

The way to solve/bypass the problem was to make Resolver a inherit from 
UserDict.

class Resolver(UserDict):
def __init__(self, data={}):
super(Resolver, self).__init__(data)
self.data["test"] = None

def __getitem__(self, name):
if (name == "test"):
return 0.5
else:
super(Resolver, self).__getitem__(name)

resolvers = (Resolver(), )
pd.eval("test", resolvers=resolvers)

I had to add a dummy "test" value to Resolver.data dict to avoid 
len(self.resolvers) to be 0. I tried to implement Resolver.__len__, keys... but 
it didn't help has ChainMap compute the len using set().union(*self.maps).

--
nosy: +ebw

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[issue30689] len() and iter() of ChainMap don't work with unhashable keys

[R. David Murray]

R. David Murray added the comment:

IMO, it would actually be surprising for ChainMap to support unhashable keys.  
In Python, as Raymond indicates, "mapping" is pretty much synonymous with 
'dict-like', and it would be where that was *not* true that we might add extra 
documentation.

--
nosy: +r.david.murray

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[issue30689] len() and iter() of ChainMap don't work with unhashable keys

[Raymond Hettinger]

Raymond Hettinger added the comment:

No thank you.  It is perfectly reasonable to use set operations to make the 
implementation simple, fast, reliable, and suitable for most use cases.  
Marking this as a rejected feature request.

As for documentation, I don't see any need to accommodate a pedantic reading of 
"mapping" in the general sense versus collections.Mapping in the formal sense.  
Additional text would likely create more confusion than it solves and would 
likely get in the way of normal users who are just trying to learn how to use 
ChainMap.

If you truly feel that users need to know about every place that the  adjective 
"mapping" carries with it a dependency on hashability, then consider a FAQ 
entry or blog post on the subject.  Otherwise, we going have to replace term 
"mapping" with the more awkward "dict-like object".

Serhiy, while I admire your devotion to studying all the code in the standard 
library, it really isn't helpful to have you challenge and second guess every 
single design decision.  While you may have a ton of spare time to generate 
enormous numbers of PRs, feature requests, and bug reports, it is eating-up 
*all* of my development time.  It is a constant distraction from real work than 
needs to be done.   In particular, most of the issues seem to be "invented 
problems" that aren't based on any actual user need or reported problem.  
Further, it risks excessive code churn, feature creep, risk of introducing new 
problems/incompatibilities, and destabilizes published code.  It may be fun to 
rewrite everything you look at, but it isn't a good practice.

--
resolution:  -> rejected
stage:  -> resolved
status: open -> closed
type: behavior -> enhancement
versions:  -Python 2.7, Python 3.5, Python 3.6

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[issue30689] len() and iter() of ChainMap don't work with unhashable keys

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

As documented ChainMap supports not just dicts, but mappings. A mapping can be 
implemented not only as a hash table, and mapping keys can be not hashable. But 
ChainMap.__len__ and ChainMap.__iter__ work only with hashable keys.

This may be considered just as a documentation issue. But I think it would be 
nice to add the support of unhashable keys in 3.7.

--
assignee: rhettinger
components: Library (Lib)
messages: 296236
nosy: rhettinger, serhiy.storchaka
priority: normal
severity: normal
status: open
title: len() and iter() of ChainMap don't work with unhashable keys
type: behavior
versions: Python 2.7, Python 3.5, Python 3.6, Python 3.7

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