What does from future import absolute_import actually do?
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
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.stringmodule, and that module is bound to the name
So I created this exact directory structure:
$ ls -R .: pkg/ ./pkg: __init__.py main.py string.py
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
$ python2.5 pkg/main.py Traceback (most recent call last): File "pkg/main.py", line 2, in <module> 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_importdirective. This absolute-import behaviour will become the default in a future version (probably Python 2.7). Once absolute imports are the default,
import stringwill 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
$ python2.5 pkg/main2.py Traceback (most recent call last): File "pkg/main2.py", line 3, in <module> 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
$ python2.7 pkg/main.py Traceback (most recent call last): File "pkg/main.py", line 2, in <module> 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 <module> print string.ascii_uppercase AttributeError: 'module' object has no attribute 'ascii_uppercase'
as well as Python 3.5, with or without (assuming the
$ python3.5 pkg/main.py Traceback (most recent call last): File "pkg/main.py", line 2, in <module> 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 <module> 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
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):
As initialized upon program startup, the first item of this list,
path, 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),
pathis 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?
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
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.