python – ImportError: Cannot import name X

The Question :

583 people think this question is useful

I have four different files named: main, vector, entity and physics. I will not post all the code, just the imports, because I think that’s where the error is. (If you want, I can post more)

Main:

import time
from entity import Ent
from vector import Vect
#the rest just creates an entity and prints the result of movement

Entity:

from vector import Vect
from physics import Physics
class Ent:
    #holds vector information and id
def tick(self, dt):
    #this is where physics changes the velocity and position vectors

Vector:

from math import *
class Vect:
    #holds i, j, k, and does vector math

Physics:

from entity import Ent
class Physics:
    #physics class gets an entity and does physics calculations on it.

I then run from main.py and I get the following error:

Traceback (most recent call last):
File "main.py", line 2, in <module>
    from entity import Ent
File ".../entity.py", line 5, in <module>
    from physics import Physics
File ".../physics.py", line 2, in <module>
    from entity import Ent
ImportError: cannot import name Ent

I am very new to Python but have worked with C++ for a long time. I’m guessing that the error is due to importing entity twice, once in main, and later in physics, but I don’t know a workaround. Can anyone help?

The Question Comments :
  • What’s the directory structure of where they are stored and in which directories?
  • have a look at this answer for loop-importing in python: stackoverflow.com/questions/7199466/…
  • In general, it’s not good coding practice to do from <module> import <name>, or from <modlue> import *. Better to import under the module namespace to prevent the chance of overwriting identically named references.
  • @jsells You should just call your classes Entity and Vector instead of Ent and Vect, there’s no reason to shorten such names. And yes, use import vector and then x = vector.Vector(0,0,0).
  • Hey @Kevin since you know Java better, what is your impression of this 2008 article where the author’s first sentence refers to how circular dependencies are “pretty common practice” in Java ?

The Answer 1

542 people think this answer is useful

You have circular dependent imports. physics.py is imported from entity before class Ent is defined and physics tries to import entity that is already initializing. Remove the dependency to physics from entity module.

The Answer 2

154 people think this answer is useful

While you should definitely avoid circular dependencies, you can defer imports in python.

for example:

import SomeModule

def someFunction(arg):
    from some.dependency import DependentClass

this ( at least in some instances ) will circumvent the error.

The Answer 3

122 people think this answer is useful

This is a circular dependency. It can be solved without any structural modifications to the code. The problem occurs because in vector you demand that entity be made available for use immediately, and vice versa. The reason for this problem is that you asking to access the contents of the module before it is ready — by using from x import y. This is essentially the same as

import x
y = x.y
del x

Python is able to detect circular dependencies and prevent the infinite loop of imports. Essentially all that happens is that an empty placeholder is created for the module (ie. it has no content). Once the circularly dependent modules are compiled it updates the imported module. This is works something like this.

a = module() # import a

# rest of module

a.update_contents(real_a)

For python to be able to work with circular dependencies you must use import x style only.

import x
class cls:
    def __init__(self):
        self.y = x.y

Since you are no longer referring to the contents of the module at the top level, python can compile the module without actually having to access the contents of the circular dependency. By top level I mean lines that will be executed during compilation as opposed to the contents of functions (eg. y = x.y). Static or class variables accessing the module contents will also cause problems.

The Answer 4

25 people think this answer is useful

To make logic clear is very important. This problem appear, because the reference become a dead loop.

If you don’t want to change the logic, you can put the some import statement which caused ImportError to the other position of file, for example the end.

a.py

from test.b import b2

def a1():
    print('a1')
    b2()

b.py

from test.a import a1

def b1():
    print('b1')
    a1()

def b2():
    print('b2')

if __name__ == '__main__':
    b1()

You will get Import Error: ImportError: cannot import name 'a1'

But if we change the position of from test.b import b2 in A like below:

a.py

def a1():
    print('a1')
    b2()

from test.b import b2

And the we can get what we want:

b1
a1
b2

The Answer 5

19 people think this answer is useful

This is a circular dependency. we can solve this problem by using import module or class or function where we needed. if we use this approach, we can fix circular dependency

A.py

from B import b2
def a1():
    print('a1')
    b2()

B.py

def b1():
   from A import a1
   print('b1')
   a1()

def b2():
   print('b2')
if __name__ == '__main__':
   b1() 

The Answer 6

17 people think this answer is useful

I just got this error too, for a different reason…

from my_sub_module import my_function

The main script had Windows line endings. my_sub_module had UNIX line endings. Changing them to be the same fixed the problem. They also need to have the same character encoding.

The Answer 7

12 people think this answer is useful

As already mentioned, this is caused by a circular dependency. What has not been mentioned is that when you’re using Python typing module and you import a class only to be used to annotate Types, you can use Forward references:

When a type hint contains names that have not been defined yet, that definition may be expressed as a string literal, to be resolved later.

and remove the dependency (the import), e.g. instead of

from my_module import Tree

def func(arg: Tree):
    # code

do:

def func(arg: 'Tree'):
    # code

(note the removed import statement)

The Answer 8

11 people think this answer is useful

The problem is clear: circular dependency between names in entity and physics modules.

Regardless of importing the whole module or just a class, the names must be loaded .

Watch this example:

# a.py
import b
def foo():
  pass
b.bar()

# b.py
import a
def bar():
  pass
a.foo()

This will be compiled into:

# a.py
# import b
# b.py
# import a # ignored, already importing
def bar():
  pass
a.foo()
# name a.foo is not defined!!!
# import b done!
def foo():
  pass
b.bar()
# done!


With one slight change we can solve this:

# a.py
def foo():
  pass
import b
b.bar()

# b.py
def bar():
  pass
import a
a.foo()

This will be compiled into:

# a.py
def foo():
  pass
# import b
# b.py
def bar():
  pass
# import a # ignored, already importing
a.foo()
# import b done!
b.bar()
# done!

The Answer 9

8 people think this answer is useful

Don’t name your current python script with the name of some other module you import

Solution: rename your working python script

Example:

  1. you are working in medicaltorch.py
  2. in that script, you have: from medicaltorch import datasets as mt_datasets where medicaltorch is supposed to be an installed module

This will fail with the ImportError. Just rename your working python script in 1.

The Answer 10

8 people think this answer is useful

In my case, I was working in a Jupyter notebook and this was happening due the import already being cached from when I had defined the class/function inside my working file.

I restarted my Jupyter kernel and the error disappeared.

The Answer 11

6 people think this answer is useful

If you are importing file1.py from file2.py and used this:

if __name__ == '__main__':
    # etc

Variables below that in file1.py cannot be imported to file2.py because __name__ does not equal __main__!

If you want to import something from file1.py to file2.py, you need to use this in file1.py:

if __name__ == 'file1':
    # etc

In case of doubt, make an assert statement to determine if __name__=='__main__'

The Answer 12

5 people think this answer is useful

Don’t see this one here yet – this is incredibly stupid, but make sure you’re importing the correct variable/function.

I was getting this error

ImportError: cannot import name IMPLICIT_WAIT

because my variable was actually IMPLICIT_TIMEOUT.

when I changed my import to use the correct name, I no longer got the error 🤦‍♂️

The Answer 13

5 people think this answer is useful

One way to track import error is step by step trying to run python on each of imported files to track down bad one.

  1. you get something like:

    python ./main.py
    
    

    ImportError: cannot import name A

  2. then you launch:

    python ./modules/a.py
    
    

    ImportError: cannot import name B

  3. then you launch:

    python ./modules/b.py
    
    

    ImportError: cannot import name C (some NON-Existing module or some other error)

The Answer 14

4 people think this answer is useful

Also not directly relevant to the OP, but failing to restart a PyCharm Python console, after adding a new object to a module, is also a great way to get a very confusing ImportError: Cannot import name ...

The confusing part is that PyCharm will autocomplete the import in the console, but the import then fails.

The Answer 15

1 people think this answer is useful

Not specifically for this asker, but this same error will show if the class name in your import doesn’t match the definition in the file you’re importing from.

The Answer 16

0 people think this answer is useful

In my case, simply missed filename:

from A.B.C import func_a (x)

from A.B.C.D import func_a (O)

where D is file.

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