Tutorial is unclear on multiple imports of a module.

[roysmith]

BPO	15575
Nosy	@ezio-melotti, @bitdancer
Files	module_multi_exec_doc_27.patch: 2.7 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 = None
closed_at = <Date 2013-04-21.21:17:06.903>
created_at = <Date 2012-08-07.18:11:02.084>
labels = ['easy', 'type-feature', 'docs']
title = 'Tutorial is unclear on multiple imports of a module.'
updated_at = <Date 2013-04-21.21:17:06.902>
user = 'https://bugs.python.org/roysmith'

bugs.python.org fields:

activity = <Date 2013-04-21.21:17:06.902>
actor = 'r.david.murray'
assignee = 'docs@python'
closed = True
closed_date = <Date 2013-04-21.21:17:06.903>
closer = 'r.david.murray'
components = ['Documentation']
creation = <Date 2012-08-07.18:11:02.084>
creator = 'roysmith'
dependencies = []
files = ['29575']
hgrepos = []
issue_num = 15575
keywords = ['patch', 'easy']
message_count = 5.0
messages = ['167628', '167629', '185256', '187524', '187525']
nosy_count = 6.0
nosy_names = ['roysmith', 'ezio.melotti', 'r.david.murray', 'docs@python', 'python-dev', 'jamesob']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue15575'
versions = ['Python 2.7']

[roysmith]

Opening this bug at Ben Finney's request. See https://groups.google.com/forum/?fromgroups#!topic/comp.lang.python/wmDUrpW2ZCU for the full thread discussing the problem. Here's a significant excerpt:

-------------------------------------------------
The tutorial is misleading on this. It it says plainly:

A module can contain executable statements as well as function 
definitions. […] They are executed only the *first* time the module 
is imported somewhere. 

<URL:http://docs.python.org/tutorial/modules.html> 

but it doesn't make clear that a module can exist in the ‘sys.modules’
list multiple times under different names.
-------------------------------------------------

Also note:

--------------------------------------------------
The footnote to that is wrong too:

[1] In fact function definitions are also ‘statements’ that are ‘executed’; the execution of a module-level function enters the function name in the module’s global symbol table.

I think what it's supposed to say is "... the execution of a module-level def statement ..."
---------------------------------------------------

[bitdancer]

Well, I don't think a full discussion of the subtlety about a module appearing under multiple names belongs in the tutorial, but I think the sentence could be amended to say "Statements in a module are executed only the *first* time the module name is encountered in an import statement".

The other is indeed a bug (IMO the sentence is missing the word 'definition', as in 'execution of a function definition').

[jamesob]

Howdy; attached is a patch containing the doc changes. I figured that some elaboration was in addition to what r.david.murray recommended, so I've included a brief allusion to the problem of equivalent paths.

[python-dev]

New changeset 9df9931fae96 by R David Murray in branch '3.3':
bpo-15575: Clarify tutorial description of when modules are executed.
http://hg.python.org/cpython/rev/9df9931fae96

New changeset dac847938326 by R David Murray in branch 'default':
bpo-15575: Clarify tutorial description of when modules are executed.
http://hg.python.org/cpython/rev/dac847938326

New changeset a1421d28393b by R David Murray in branch '2.7':
bpo-15575: Clarify tutorial description of when modules are executed.
http://hg.python.org/cpython/rev/a1421d28393b

[bitdancer]

Thanks, James. I wound up going with a different wording for the "elaboration": since the concept of running a python file as a script is mentioned just a bit earlier, I added a parenthetical that the statements are also executed if the module is run as a script.