What does from future import absolute import actually do

Question

I have answered a question regarding absolute imports in Python  which I thought I understood based on reading the Python 2 5 changelog and accompanying PEP  However  upon installing Python 2 5 and attempting to craft an example of properly using from   future   import absolute import  I realize things are not so clear   Straight from the changelog linked above  this statement accurately summarized my understanding of the absolute import change      Let s say you have a package directory like this   pkg  pkg   init   py pkg main py pkg string py       This defines a package named pkg containing the pkg main and pkg string submodules       Consider the code in the main py module  What happens if it executes the statement import string  In Python 2 4 and earlier  it will first look in the package s directory to perform a relative import  finds pkg string py  imports the contents of that file as the pkg string module  and that module is bound to the name  string  in the pkg main module s namespace    So I created this exact directory structure     ls -R    pkg     pkg    init   py  main py  string py     init   py and string py are empty  main py contains the following code   import string print string ascii uppercase   As expected  running this with Python 2 5 fails with an AttributeError     python2 5 pkg main py Traceback  most recent call last     File  pkg main py   line 2  in  lt module gt      print string ascii uppercase AttributeError   module  object has no attribute  ascii uppercase    However  further along in the 2 5 changelog  we find this  emphasis added       In Python 2 5  you can switch import s behaviour to absolute imports using a from   future   import absolute import directive  This absolute-import behaviour will become the default in a future version  probably Python 2 7   Once absolute imports are the default  import string will always find the standard library s version    I thus created pkg main2 py  identical to main py but with the additional future import directive  It now looks like this   from   future   import absolute import import string print string ascii uppercase   Running this with Python 2 5  however    fails with an AttributeError     python2 5 pkg main2 py Traceback  most recent call last     File  pkg main2 py   line 3  in  lt module gt      print string ascii uppercase AttributeError   module  object has no attribute  ascii uppercase    This pretty flatly contradicts the statement that import string will always find the std-lib version with absolute imports enabled  What s more  despite the warning that absolute imports are scheduled to become the  new default  behavior  I hit this same problem using both Python 2 7  with or without the   future   directive     python2 7 pkg main py Traceback  most recent call last     File  pkg main py   line 2  in  lt module gt      print string ascii uppercase AttributeError   module  object has no attribute  ascii uppercase     python2 7 pkg main2 py Traceback  most recent call last     File  pkg main2 py   line 3  in  lt module gt      print string ascii uppercase AttributeError   module  object has no attribute  ascii uppercase    as well as Python 3 5  with or without  assuming the print statement is changed in both files      python3 5 pkg main py Traceback  most recent call last     File  pkg main py   line 2  in  lt module gt      print string ascii uppercase  AttributeError  module  string  has no attribute  ascii uppercase     python3 5 pkg main2 py Traceback  most recent call last     File  pkg main2 py   line 3  in  lt module gt      print string ascii uppercase  AttributeError  module  string  has no attribute  ascii uppercase      I have tested other variations of this  Instead of string py  I have created an empty module -- a directory named string containing only an empty   init   py -- and instead of issuing imports from main py  I have cd d to pkg and run imports directly from the REPL  Neither of these variations  nor a combination of them  changed the results above  I cannot reconcile this with what I have read about the   future   directive and absolute imports   It seems to me that this is easily explicable by the following  this is from the Python 2 docs but this statement remains unchanged in the same docs for Python 3       sys path                 As initialized upon program startup  the first item of this list  path 0   is the directory containing the script that was used to invoke the Python interpreter  If the script directory is not available  e g  if the interpreter is invoked interactively or if the script is read from standard input   path 0  is the empty string  which directs Python to search modules in the current directory first    So what am I missing  Why does the   future   statement seemingly not do what it says  and what is the resolution of this contradiction between these two sections of documentation  as well as between described and actual behavior

User · Answer

The difference between absolute and relative imports come into play only when you import a module from a package and that module imports an other submodule from that package. See the difference:

$ mkdir pkg
$ touch pkg/__init__.py
$ touch pkg/string.py
$ echo 'import string;print(string.ascii_uppercase)' > pkg/main1.py
$ python2
Python 2.7.9 (default, Dec 13 2014, 18:02:08) [GCC] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pkg.main1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pkg/main1.py", line 1, in <module>
    import string;print(string.ascii_uppercase)
AttributeError: 'module' object has no attribute 'ascii_uppercase'
>>> 
$ echo 'from __future__ import absolute_import;import string;print(string.ascii_uppercase)' > pkg/main2.py
$ python2
Python 2.7.9 (default, Dec 13 2014, 18:02:08) [GCC] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pkg.main2
ABCDEFGHIJKLMNOPQRSTUVWXYZ
>>>

In particular:

$ python2 pkg/main2.py
Traceback (most recent call last):
  File "pkg/main2.py", line 1, in <module>
    from __future__ import absolute_import;import string;print(string.ascii_uppercase)
AttributeError: 'module' object has no attribute 'ascii_uppercase'
$ python2
Python 2.7.9 (default, Dec 13 2014, 18:02:08) [GCC] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pkg.main2
ABCDEFGHIJKLMNOPQRSTUVWXYZ
>>> 
$ python2 -m pkg.main2
ABCDEFGHIJKLMNOPQRSTUVWXYZ

Note that python2 pkg/main2.py has a different behaviour then launching python2 and then importing pkg.main2 (which is equivalent to using the -m switch).

If you ever want to run a submodule of a package always use the -m switch which prevents the interpreter for chaining the sys.path list and correctly handles the semantics of the submodule.

Also, I much prefer using explicit relative imports for package submodules since they provide more semantics and better error messages in case of failure.

User · Answer

The changelog is sloppily worded  from   future   import absolute import does not care about whether something is part of the standard library  and import string will not always give you the standard-library module with absolute imports on   from   future   import absolute import means that if you import string  Python will always look for a top-level string module  rather than current package string  However  it does not affect the logic Python uses to decide what file is the string module  When you do  python pkg script py   pkg script py doesn t look like part of a package to Python  Following the normal procedures  the pkg directory is added to the path  and all  py files in the pkg directory look like top-level modules  import string finds pkg string py not because it s doing a relative import  but because pkg string py appears to be the top-level module string  The fact that this isn t the standard-library string module doesn t come up   To run the file as part of the pkg package  you could do  python -m pkg script   In this case  the pkg directory will not be added to the path  However  the current directory will be added to the path   You can also add some boilerplate to pkg script py to make Python treat it as part of the pkg package even when run as a file   if   name         main    and   package   is None        package      pkg    However  this won t affect sys path  You ll need some additional handling to remove the pkg directory from the path  and if pkg s parent directory isn t on the path  you ll need to stick that on the path too

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