Collections - ChainMap - Documentation example wrong order line

[MarcosSoutullo]

BPO	31075
Nosy	@rhettinger

^{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 2017-09-07.21:26:09.399>
created_at = <Date 2017-07-29.12:24:17.171>
labels = ['build', 'invalid', 'docs']
title = 'Collections - ChainMap - Documentation example wrong order line'
updated_at = <Date 2017-09-07.21:26:09.398>
user = 'https://bugs.python.org/MarcosSoutullo'

bugs.python.org fields:

activity = <Date 2017-09-07.21:26:09.398>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2017-09-07.21:26:09.399>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2017-07-29.12:24:17.171>
creator = 'Marcos Soutullo'
dependencies = []
files = []
hgrepos = []
issue_num = 31075
keywords = []
message_count = 3.0
messages = ['299465', '299478', '299484']
nosy_count = 3.0
nosy_names = ['rhettinger', 'docs@python', 'Marcos Soutullo']
pr_nums = []
priority = 'low'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue31075'
versions = ['Python 3.6']

[MarcosSoutullo]

Hello,

I have been taking a look into your ChainMap collections documentation (https://docs.python.org/3.6/library/collections.html?highlight=collections#collections.ChainMap), specifically the third code example on "8.3.1.1. ChainMap Examples and Recipes"
that clearly describe and illustrate a use case for the ChainMap class. However, I found a very small code issue in regards to how the sample code (line 7) is presented to the reader. Please refer to the code compilation below:

Line 7 >>> d['x']                # Get first key in the chain of contexts
raise KeyError(key)
KeyError: 'x'
Line 8 >>> d['x'] d['x'] = 1            # Set value in current context

The key named 'x' had not been initialised yet and of course, the compiler is complaining. It should be the other way around.

Line 8 >>> d['x'] = 1   # Set value in current context
Line 7 >>> d['x']       # Get first key in the chain of contexts

Many thanks for the great work you do with Python,

Marcos S.

[rhettinger]

-0

The code block was originally intended to just be a table of patterns, not meant to be executed sequentially. It followed the traditional presentation order (getitem, __setitem__, followed by __delitem__).

If we do change this, then the subsequent "k in d" example should also be changed to "'x' in d" because k isn't defined.

[rhettinger]

Many thanks for the great work you do with Python,

Thanks for the kudos. Also, thanks for the close reading of the docs.