Use 'import module' or 'from module im

Question

I've tried to find a comprehensive guide on whether it is best to use import module or from module import? I've just started with Python and I'm trying to start off with best practices in mind.

Basically, I was hoping if anyone could share their experiences, what preferences other developers have and what's the best way to avoid any gotchas down the road?

Answer 1

Even though many people already explained about import vs import from, I want to try to explain a bit more about what happens under the hood, and where all the places it changes are.

---

import foo:

Imports foo, and creates a reference to that module in the current namespace. Then you need to define completed module path to access a particular attribute or method from inside the module.

E.g. foo.bar but not bar

from foo import bar:

Imports foo, and creates references to all the members listed (bar). Does not set the variable foo.

E.g. bar but not baz or foo.baz

from foo import *:

Imports foo, and creates references to all public objects defined by that module in the current namespace (everything listed in __all__ if __all__ exists, otherwise everything that doesn't start with _). Does not set the variable foo.

E.g. bar and baz but not _qux or foo._qux.

---

Now let’s see when we do import X.Y:

>>> import sys
>>> import os.path

Check sys.modules with name os and os.path:

>>> sys.modules['os']
<module 'os' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/os.pyc'>
>>> sys.modules['os.path']
<module 'posixpath' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>

Check globals() and locals() namespace dicts with os and os.path:

 >>> globals()['os']
<module 'os' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/os.pyc'>
>>> locals()['os']
<module 'os' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/os.pyc'>
>>> globals()['os.path']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'os.path'
>>>

From the above example we found that only os is inserted in the local and global namespace. So, we should be able to use:

 >>> os
 <module 'os' from
  '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/os.pyc'>
 >>> os.path
 <module 'posixpath' from
 '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>
 >>>

But not path.

>>> path
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'path' is not defined
>>>

Once you delete the os from locals() namespace, you won't be able to access os as well as os.path even though they exist in sys.modules:

>>> del locals()['os']
>>> os
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined
>>> os.path
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined
>>>

Now let's talk about import from:

from:

>>> import sys
>>> from os import path

Check sys.modules with os and os.path:

>>> sys.modules['os']
<module 'os' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/os.pyc'>
>>> sys.modules['os.path']
<module 'posixpath' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>

We found that in sys.modules we found as same as we did before by using import name

OK, let's check how it looks like in locals() and globals() namespace dicts:

>>> globals()['path']
<module 'posixpath' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>
>>> locals()['path']
<module 'posixpath' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>
>>> globals()['os']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'os'
>>>

You can access by using name path not by os.path:

>>> path
<module 'posixpath' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>
>>> os.path
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined
>>>

Let's delete 'path' from locals():

>>> del locals()['path']
>>> path
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'path' is not defined
>>>

One final example using an alias:

>>> from os import path as HELL_BOY
>>> locals()['HELL_BOY']
<module 'posixpath' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>
>>> globals()['HELL_BOY']
<module 'posixpath' from /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.pyc'>
>>>

And no path defined:

>>> globals()['path']
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
KeyError: 'path'
>>>