ImportError: Cannot import name X

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Solving problem is about exposing yourself to as many situations as possible like ImportError: Cannot import name X and practice these strategies over and over. With time, it becomes second nature and a natural way you approach any problems in general. Big or small, always start with a plan, use other strategies mentioned here till you are confident and ready to code the solution.
In this post, my aim is to share an overview the topic about ImportError: Cannot import name X, which can be followed any time. Take easy to follow this discuss.

ImportError: Cannot import name X

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?

Asked By: jsells

||

Answer #1:

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.

Answered By: Teemu Ikonen

Answer #2:

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.

Answered By: bharling

Answer #3:

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.

Answered By: Dunes

Answer #4:

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
Answered By: g10guang

Answer #5:

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()
Answered By: a.patidar

Answer #6:

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.

Answered By: marengaz

Answer #7:

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)

Answered By: Tomasz Bartkowiak

Answer #8:

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!
Answered By: DuniC

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