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I have a folder for my client code, a folder for my server code, and a folder for code that is shared between them

Proj/
    Client/
        Client.py
    Server/
        Server.py
    Common/
        __init__.py
        Common.py

How do I import Common.py from Server.py and Client.py?

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7 Answers 7

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228

EDIT Nov 2014 (3 years later):

Python 2.6 and 3.x supports proper relative imports, where you can avoid doing anything hacky. With this method, you know you are getting a relative import rather than an absolute import. The '..' means, go to the directory above me:

from ..Common import Common

As a caveat, this will only work if you run your python as a module, from outside of the package. For example:

python -m Proj

Original hacky way

This method is still commonly used in some situations, where you aren't actually ever 'installing' your package. For example, it's popular with Django users.

You can add Common/ to your sys.path (the list of paths python looks at to import things):

import sys, os
sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'Common'))
import Common

os.path.dirname(__file__) just gives you the directory that your current python file is in, and then we navigate to 'Common/' the directory and import 'Common' the module.

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7 Comments

Do not modify python modules path manually, may be only for quick hacks. Learning Python package management using distutils, setuptools etc. is usually a required skill that will solve problems like that.
@SaschaGottfried totally agree, although if you're not making a distributable package, it probably won't matter. For example, in Django you never really install your app with distutils, so the above method is an easy hack. But anyways I've edited the answer with what I would do these days.
Thanks for answering the actual question instead of preaching about proper technique. There are plenty of good reasons to do relative imports.
to go up one more level, use an additional dot for each level. @jxramos ex: from ...myfile goes to ../../myfile
@WattsInABox how would you go up and go to a file in a different directory, say the equivalent of ../../mydir2/myfile?
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13

Funny enough, a same problem I just met, and I get this work in following way:

combining with linux command ln , we can make thing a lot simper:

1. cd Proj/Client
2. ln -s ../Common ./

3. cd Proj/Server
4. ln -s ../Common ./

And, now if you want to import some_stuff from file: Proj/Common/Common.py into your file: Proj/Client/Client.py, just like this:

# in Proj/Client/Client.py
from Common.Common import some_stuff

And, the same applies to Proj/Server, Also works for setup.py process, a same question discussed here, hope it helps !

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1 Comment

Someone did that. But when I cloned their code on windows, instead of symbolic link I had common.py file with ../Common string inside.
12

Doing a relative import is absolulutely OK! Here's what little 'ol me does:

#first change the cwd to the script path
scriptPath = os.path.realpath(os.path.dirname(sys.argv[0]))
os.chdir(scriptPath)

#append the relative location you want to import from
sys.path.append("../common")

#import your module stored in '../common'
import common.py
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3 Comments

But you better know where sys.argv[0] is actually pointing - it (prolly) isn't the directory you were in when you started python.
This is a quick hack, with lot of pitfalls. But the question was not even better.
This clearly written, but the original hack in Dave's answer is better because it uses __file__ to get the proper relation from the current file
9

Don't do relative import.

From PEP8:

Relative imports for intra-package imports are highly discouraged.

Put all your code into one super package (i.e. "myapp") and use subpackages for client, server and common code.

Update: "Python 2.6 and 3.x supports proper relative imports (...)". See Dave's answers for more details.

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7 Comments

Imagine that you add some code to the end of the client and server after the 'if __name__ == "__main__":' line. That is, you want to be able to use them as stand-alone scripts. How to do it properly? I think it's a perfectly common use case that should be supported. Why is it discouraged?
I am surprised that "Don't do it" is the accepted answer for a "how do I..." question (well, except for Rails <g>.) There are occasional reasons to do this. I use a solution similar to what Dave suggests.
@TomWilson: It's not pure "don't do it" answer. There is "do it this way" below.
Someone should tell the guys over at Numpy! They use a TON of relative imports!
This answer is not applicable to current versions of Python. The quoted part is no longer to be found in PEP 8. Nowadays it reads like: "explicit relative imports are an acceptable alternative to absolute imports, especially when dealing with complex package layouts where using absolute imports would be unnecessarily verbose"
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6

The default import method is already "relative", from the PYTHONPATH. The PYTHONPATH is by default, to some system libraries along with the folder of the original source file. If you run with -m to run a module, the current directory gets added to the PYTHONPATH. So if the entry point of your program is inside of Proj, then using import Common.Common should work inside both Server.py and Client.py.

Don't do a relative import. It won't work how you want it to.

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1 Comment

If this is true, why are the top answers not saying this? Will this work or not?
3

Approch used by me is similar to Gary Beardsley mentioned above with small change.

Filename: Server.py

import os, sys
script_path = os.path.realpath(os.path.dirname(__name__))
os.chdir(script_path)
sys.path.append("..")
# above mentioned steps will make 1 level up module available for import
# here Client, Server and Common all 3 can be imported.

# below mentioned import will be relative to root project
from Common import Common
from Client import Client
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2

Create a simple example

Assume we run ls -R in the current working directory and this is the result:

./second_karma:
enemy.py  import.py  __init__.py  math

./second_karma/math:
fibonacci.py  __init__.py

And we run this command $ python3 second-karma/import.py

init.py is an empty file but it should exists.

Now let's see what is inside the second-karma/import.py:

from .math.fibonacci import Fibonacci
fib = Fibonacci()
print(fib.get_fibonacci(15))

And what is inside the second_karma/math/fibonacci.py:

from ..enemy import Enemy
class Fibonacci:
    enemy: Enemy

    def __init__(self):
        self.enemy = Enemy(150,900)
        print("Class instantiated")
    
    def get_fibonacci(self, which_index: int) -> int:
        print(self.enemy.get_hp())
        return 4

Now the last file is second_karma/enemy.py:

class Enemy:
    hp: int = 100
    attack_low: int = 180
    attack_high: int = 360

    def __init__(
            self, 
            attack_low: int,
            attack_high: int) -> None: 
        self.attack_low = attack_low
        self.attack_high = attack_high

    def getAttackPower(
            self) -> {"attack_low": int, "attack_high": int}:
        return {
            "attack_low": self.attack_low,
            "attack_high": self.attack_high
        }

    def get_hp(self) -> int:
        return self.hp

Now a simple answer why it was not working:

  • Python has a concept of packages, which is basically a folder containing one or more modules, and zero-or-more packages.
  • When we launch python, there are two ways of doing it:
    • Asking python to execute a specific module (python3 path/to/file.py).
    • Asking python to execute a package.
  • The issue is that import.py makes reference to importing .math
    • The .math in this context means "go find a module/package in the current package with the name math"
    • Trouble:
      • When I execute as $ python3 second-karma/import.py I am executing a module, not a package. thus python has no idea what . means in this context
      • Fix: 
        python3 -m second_karma.import
      • Now import.py is of parent package second_karma, and thus your relative import will work.

Important note:

Those __init__.py are necessary and if you have not them you must create them first.

An example in github

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