Why my init.py doesn't work in the module?

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?

It's unclear why you expected that would work. Declaring a variable in __init__.py doesn't make it in scope for every other file in the directory. I'd recommend reading e.g. docs.python.org/3/tutorial/modules.html. — jonrsharpe
– jonrsharpe, Commented Oct 22, 2019 at 10:07
@jonrsharpe Thank you, I will take a look at the link first. I want that work because I don't want to define project_root in every file. — John Yul
– John Yul, Commented Oct 22, 2019 at 10:38
@jonrsharpe because I have to operate csv files in another dir, so I need to use project_root path to generate csv file path. — John Yul
– John Yul, Commented Oct 22, 2019 at 10:42

Amit Yadav · Accepted Answer · 2019-10-22 10:53:54Z

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 : )

edited Oct 22, 2019 at 10:53

answered Oct 22, 2019 at 10:36

Amit Yadav

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1 Comment

John Yul Over a year ago

Thanks @Amit Yadav, I understand your example.So if I want to use the project_root argument inside the package (files in dir), __init__.py doesn't work because it won't be execute while I execute file in the dir.

micric · Accepted Answer · 2019-10-22 10:16:43Z

0

You have to import the variable:

from dir import project_root

Where dir is the directory you are in

answered Oct 22, 2019 at 10:16

micric

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1 Comment

John Yul Over a year ago

I tried, suppose the dir is tmp, I got the error ``` Traceback (most recent call last): File "a.py", line 1, in <module> from tmp import project_root ModuleNotFoundError: No module named 'tmp' ```

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