为什么我的 __init__.py 在模块中不起作用？

Question

I am trying to add 'project_root' into __init__.py and all modules can use it, but it doesn't work.

Environment: Python 3.7.0 MACOS MOJAVE

file structure

·
├── __init__.py
└── a.py

the codes in __init__.py file:

import sys

project_root = os.path.dirname(os.path.dirname(os.path.realpath(__file__)))
sys.path.append(project_root)

and in another file

print(project_root)

If I run python a.py in the same dir,or run python a.py out of the dir, error are the same below:

Traceback (most recent call last):
  File "a.py", line 1, in <module>
    print(project_root)
NameError: name 'project_root' is not defined

My question is why it doesn't work and how to fix it. Another question is what if you want to share some variables for other modules in a same package, how to do it?

Answer 1

Let us try to understand by example.

Code and directory explanation:

Suppose we have the following directory and file structure:

dir_1
    ├── __init__.py
    └── a.py
b.py

__init__.py contains:

import sys,os

# Just to make things clear
print("Print statement from init")

project_root = os.path.dirname(os.path.dirname(os.path.realpath(__file__)))
sys.path.append(project_root)

a.py contains:

def func():
    print("func from a.py")

Let us start importing things:

Suppose you start with having the following code in b.py :

from dir_1.a import func

func()

Executing the above will give the following output:

Print statement from init
func from a.py

So, from the above, we understand that the print statement from __init__.py is being executed. Now, let's add print(project_root) in b.py :

from dir_1.a import func

func()
print(project_root)

Executing the above will result in an error saying:

...
NameError: name 'project_root' is not defined

This happened because we did not have to import the print statement from __init__.py it just gets executed. But that is not the case for a variable.

Let's try to import the variable and see what happens:

from dir_1.a import func
from dir_1 import project_root

func()
print(project_root)

Executing the above file will give the following output:

Print statement from init
func from a.py
/home/user/some/directory/name/dir_1

Long story short , you need to import the variable defined in __init__.py or anywhere else in order to use it.

Hope this helps: )

Answer 2

You have to import the variable:

from dir import project_root

Where dir is the directory you are in