On Fri, Sep 13, 2013 at 3:58 PM, Saager Mhatre <saager.mha...@gmail.com> wrote: > On Fri, Sep 13, 2013 at 1:10 PM, Saju M <sajup...@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!" > _______________________________________________ > BangPypers mailing list > BangPypers@python.org > https://mail.python.org/mailman/listinfo/bangpypers -- ---------------------------------------------------------------------------------------------------------------------------------- http://blog.dhananjaynene.com twitter: @dnene google plus: http://gplus.to/dhananjaynene _______________________________________________ BangPypers mailing list BangPypers@python.org https://mail.python.org/mailman/listinfo/bangpypers
