[BangPypers] Python packages and modules (was: Favorite tips/techniques)

[Dhananjay Nene]

On Fri, Sep 13, 2013 at 3:58 PM, Saager Mhatre <saager.mhatre at gmail.com> wrote:
> On Fri, Sep 13, 2013 at 1:10 PM, Saju M <sajuptpm at gmail.com> wrote:
>
>> Saager,
>>
>> As per python module-import semantics, sub-modules don't end up as names
>> in the
>> package-module[*] unless explicitly added.
>>
>> I didn't get it, what you mean by package-module[*] ?.
>>
>
> Ah, there it is... the sound of your head exploding! :P
>
> Buckle up, this is going to be a fast, but rough ride!
>
> Basically...
> ----
> * A module is a dict-like object that binds names to values.
> * A package is a namespace that can contain (only) other modules (which
> could, in turn, be package-modules themselves).
> * A package-module[1] would be a module that also serves as a package.
>
> I guess it'd be easier to explain with with an example.
>
> Semantically...
> ---
> * Lets stick to json.tool. In this case, json is a module and tool is a
> sub-module of the json module.
> * They are both modules in that each can contain bindings to names =>
> json.dump and tool.main.
> * But, json is also a package in that it contains the tool module =>
> json.tool
> * The sub-module relationship is mostly evident from the fact that the tool
> module is referenced by prefixing 'json.' to it => import json.tool;
> * Or providing 'json' as the package to look for the module => from json
> import tool
>
> Physically...
> ---
> * Any .py file can be loaded as a module.
> * Any directory with an __init__.py file can be treated as a package.
> * The __init__.py file itself serves as the package-module, i.e., the
> module with same name as the package
> * Any .py files inside the directory (except __init__.py, of course) can be
> loaded as sub-modules of the above package.
> * Any sub-directories inside the directory (containing __init__.py, of
> course) can be loaded as sub-packages of the above package.
> * Turtles all the way...
>
> Funda-mentally...
> ---
> * The confusion basically stems from the fact that Python chose to conflate
> physical storage and namespacing with just enough overlap to be
> inconsistent.
> * They are conflated in that package/module naming and their lookup
> (finding the code for a module) is tied to the physical storage hierarchy.
> * They are inconsistent that module loading is transitive only upwards in
> the hierarchy, i.e., when you load a module, all packages above it in the
> hierarchy are automatically loaded.[2]
> * However, sub-modules are not loaded even though the physical hierarchy
> evidences it.
> * The conflation extends further as we look as modules as namespaces,
> because sub-modules do not end up as names in package-modules until they
> are loaded; see below
> Python 2.7.4 (default, Apr 19 2013, 18:28:01)
> [GCC 4.7.3] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> *>>> json*
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> NameError: name 'json' is not defined
> *>>> import json*
> *>>> json*
> <module 'json' from '/usr/lib/python2.7/json/__init__.py'>
> *>>> json.tool*
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'module' object has no attribute 'tool'
> *>>> from json import tool*
> *>>> json*
> <module 'json' from '/usr/lib/python2.7/json/__init__.py'>
> *>>> tool*
> <module 'json.tool' from '/usr/lib/python2.7/json/tool.py'>
> *>>> json.tool*
> <module 'json.tool' from '/usr/lib/python2.7/json/tool.py'>
> *>>> *
>
> Finally...
> ---
> On a closing note, it goes without saying that these packages are not to be
> confused with packages as published on package indexes such as
> https://pypi.python.org.[3]

Nice. I reached out for the "Favourite" button but realised
emails/mailing lists don't have it.

I do think it can be a little easier to understand by doing
s/package-module/namespace/g. Having done that I realised a few things
needed to be reworded, and then noticed a few minor corrections were
required.

Here's the result (Have attempted to only make minor structural
corrections without adjusting any of the opinions or rationale
expressed herein)
---------------------------------------------------------------------------------------------------------
Ah, there it is... the sound of your head exploding! :P

Buckle up, this is going to be a fast, but rough ride!

Basically...
----
* A namespace is a dict-like object that binds names to values.
* A namespace is an abstract notion, which is implemented as packages or modules
  Thus a package is-a namespace and a module is-a namespace
* The package namespace includes the modules in it and the stuff in __init__.py

I guess it'd be easier to explain with with an example.

Semantically...
---
* Lets stick to json.tool. In this case, json is a package and tool is
a module
* They are both namespaces in that each can contain bindings to names
=> json.dump and tool.main.
* json is a package namespace in that it contains the tool module => json.tool
* The sub-module relationship is mostly evident from the fact that the
tool module is referenced by prefixing 'json.' to it => import
json.tool;
* Or providing 'json' as the package to look for the module => from
json import tool

Physically...
---
* Any .py file can be loaded as a module.
* Any directory with an __init__.py file can be treated as a package.
* The __init__.py file itself serves as part of the package namespace,
i.e., ie. it has the same name as the package
* Any .py files inside the directory (except __init__.py, of course)
can be loaded as sub-modules of the above package.
* Any sub-directories inside the directory (containing __init__.py, of
course) can be loaded as sub-packages of the above package.
* Turtles all the way...

Funda-mentally...
---
* The confusion basically stems from the fact that Python chose to
conflate physical storage and namespacing with just enough overlap to
be inconsistent.
* They are conflated in that package/module naming and their lookup
(finding the code for a module) is tied to the physical storage
hierarchy.
* They are inconsistent that module loading is transitive only upwards
in the hierarchy, i.e., when you load a module, all packages above it
in the hierarchy are automatically loaded.[2]
* However, sub-modules are not loaded even though the physical
hierarchy evidences it.
* The conflation extends further as we look as packages as namespaces,
because sub-modules do not end up as names in the package namespace
until they are loaded; see below
Python 2.7.4 (default, Apr 19 2013, 18:28:01)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
*>>> json*
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'json' is not defined
*>>> import json*
*>>> json*
<module 'json' from '/usr/lib/python2.7/json/__init__.py'>
*>>> json.tool*
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'tool'
*>>> from json import tool*
*>>> json*
<module 'json' from '/usr/lib/python2.7/json/__init__.py'>
*>>> tool*

>
> (steps behind transparent blast shield to calmly enjoy the sight of more
> exploding heads)
> - d
>
> [1] I don't believe it would be entirely in the best interest of either of
> our healths to use this term outside of this thread!
> [2] IIRC, this was not true for python <2.5 (I think); I recall hitting
> some really weird import errors when running newer code on a really old
> interpreter when it suddenly couldn't reference the packages that a loaded
> module belonged to until they were explicitly loaded.
> [3] To those in the know, I would be tremendously obliged if you could tell
> me what brand of blow they were using when they came up with this
> byzantine, labyrinthine nomenclature and related implementation. I bet it
> was, as they say, "like, totally radical dude!"
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