operator.setitem example no longer works in Python 3 due to lazy map

[mwilliamson]

BPO	22180
Nosy	@rhettinger, @pitrou, @bitdancer
Files	doc-operator-example.patch

^{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/rhettinger'
closed_at = <Date 2014-08-10.17:35:07.156>
created_at = <Date 2014-08-10.14:18:24.747>
labels = ['docs']
title = 'operator.setitem example no longer works in Python 3 due to lazy map'
updated_at = <Date 2014-08-10.17:35:07.155>
user = 'https://bugs.python.org/mwilliamson'

bugs.python.org fields:

activity = <Date 2014-08-10.17:35:07.155>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2014-08-10.17:35:07.156>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2014-08-10.14:18:24.747>
creator = 'mwilliamson'
dependencies = []
files = ['36333']
hgrepos = []
issue_num = 22180
keywords = ['patch']
message_count = 6.0
messages = ['225141', '225142', '225145', '225146', '225147', '225149']
nosy_count = 6.0
nosy_names = ['rhettinger', 'pitrou', 'r.david.murray', 'docs@python', 'python-dev', 'mwilliamson']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue22180'
versions = ['Python 3.4', 'Python 3.5']

[mwilliamson]

The Python docs for the operator module include an example using map and setitem to "Build a dictionary that maps the ordinals from 0 to 255 to their character equivalents.":

    d = {}
    keys = range(256)
    vals = map(chr, keys)
    map(operator.setitem, [d]*len(keys), keys, vals)   

Since map is lazy since Python 3, the dictionary d is never actually changed in this example. I'm not entirely sure what the idiomatic way to fix the example is since it strikes me as being fairly unidiomatic to begin with, but the simplest would be to call list on the result of map to force evaluation (patch attached).

[bitdancer]

Heh. There was a discussion in bpo-22106 about valid examples for using 'pass'. This case is analogous to the one I came up with.

  for x in map(...):
     pass

that avoids building a list.

Not that any of it is idiomatic, as you say.

[pitrou]

The whole example is bad and should be removed or replace with something else:

1. Using map() is an anti-pattern here, since the function results are not used.

2. To "build a dictionary that maps the ordinals from 0 to 255 to their character equivalents", modern Python code should use a dict comprehension and avoid operator.setitem() entirely.

[rhettinger]

The whole example is bad and should be removed
or replace with something else:

I'm not sure there are ANY examples of operator.setitem that couldn't be done a better way without it. How about we remove it and leave the examples for things that people ought to be doing.

[pitrou]

Le 10/08/2014 13:20, Raymond Hettinger a écrit :

Raymond Hettinger added the comment:

> The whole example is bad and should be removed
> or replace with something else:

I'm not sure there are ANY examples of operator.setitem that
couldn't
be done a better way without it. How about we remove it and leave the
examples for things that people ought to be doing.

Agreed. operator.setitem() isn't really ever used in my experience.

[python-dev]

New changeset 9c250f34bfa3 by Raymond Hettinger in branch '3.4':
Issue bpo-22180: Remove weak example
http://hg.python.org/cpython/rev/9c250f34bfa3