I am trying to have an imported module be able to access code from the module being run, but am running into an issue where the module is only partially imported. An example of what I mean would be easier to understand. Have two files in the same folder called a.py and b.py.
In a.py:
from importlib import import_module
def do_a_stuff() -> None:
print("Doing stuff in A...")
MODULE_B = import_module("b")
MODULE_B.do_a_and_b_stuff()
In b.py:
import a
def do_a_and_b_stuff() -> None:
a.do_a_stuff()
print("Doing stuff in B...")
What I want to happen is for a.py to print
Doing stuff in A...
Doing stuff in B...
But when I run this code, I get a circular reference error. I understand why this happens, but want to know how I can make this work the way I want it to.
I've tried importing b multiple times in one loop, but that doesn't change anything. Also, b cannot have a static import as my application must be able to dynamically chose what file to import at runtime based on user input.
When you import a module for the first time, regardless of using the import statement or the importlib methods, it's like executing the top-level code of the module.
This means that when in b you import a and it imports b back, Python will detect a circular reference loop and break.
After a module has been successfully imported, further imports will only fetch a cached version of the module. This means that the following code works
a.py
import b
def do_a_something():
print("a")
b.do_b_something()
b.py
def do_b_something():
import a
a.do_a_something()
print("b")
Generally you shouldn't be importing like this because the import runs on each function call rather than just once when the module is loaded, but it's a workaround that works well for the scenario you described. The initial import b will not trigger the import a, so you'll get both modules a and b loaded. Further imports (like the one inside the method) will provide the already loaded version.
You can actually manage import flow through clever use of __main__ and if statements. For this example:
In a.py:
from importlib import import_module
def do_a_stuff() -> None:
print("Doing stuff in A...")
if __name__ == "__main__":
MODULE_B = import_module("b")
MODULE_B.do_a_and_b_stuff()
In b.py:
import a
def do_a_and_b_stuff() -> None:
a.do_a_stuff()
print("Doing stuff in B...")
In this instance, a good way to do it is to create a new file within the same directory called c.py which contains the following:
import a
In this case, run c.py instead of a.py. The main benefit of this solution is that b.py can access dynamic elements of a.py.
How do I import and run a Python function and have all the dependencies it uses use the imports from the main Python file?
Main Python file:
from im import er
import time
er()
Python file with function to be imported:
def er():
time.sleep(1)
print('hi')
This doesn't work because the time module is not imported in the im.py. How can I make this work without importing the modules it needs each time I run the function?
You have to import the function in the main, and required module for the function in the function file.
Main Python file:
from im import er
er()
Imported module :
from time import sleep
def er():
sleep(1)
print('hi')
This behave this way because Python run the imported module when you import it. Then, depending of your import statement, it will do:
import <module>: create a module object named <module> with attributes allowing you to access global scope of this module. If a function is in the global scope of the module, you access it with <module>.<function_name>.
from <module> import *: add to the global scope of the current module the global scope of the imported module (not exactly, but if you need more informations about it, look for wildcard import behaviour). As a good rule of thumb with this import: don't use this.
from <module> import <symbol>: add the symbol of the imported module to the global scope of the current module.
More info about imports:
https://stackoverflow.com/a/10501768/6251742
https://docs.python.org/3/reference/import.html
I would like to import a function foo() from module abc.py
However, abc.py contains other functions which rely on modules which are not available for Python (i.e. I cannot import them into python interpreter, because I use ImageJ to run abc.py as Jython)
One solution I found is to put the problematic imports inside the name == "main" check, such as:
# abc.py
def foo():
print("Hello, World!")
def run_main_function():
foo()
...other stuff using IJ...
if __name__ == "__main__":
from ij import IJ
run_main_function()
So when I try to import foo from into another script def.py, e.g.:
# def.py
from abc import foo
def other_func():
foo()
if __name__ == "__main__":
other_func()
This works. But when I put imports in normal fashion, at the top of the script, I get an error: No module named 'ij'. I would like to know if there is a solution to this problem? Specifically, that I put the imports at the top of the script and then within def.py I say to import just the function, without dependencies of abc.py?
I would like to know if there is a solution to this problem? Specifically, that I put the imports at the top of the script and then within def.py I say to import just the function, without dependencies of abc.py?
As far I know, it's the way that python works. You should put that import in the function that uses it if won't be aviable always.
def run_main_function():
from ij import IJ
foo()
Also, don't use abc as a module name, it's a standard library module: Abstract Base Class 2.7, Abstract Base Class 3.6
Edit: don't use trailing .py when importing as Kind Stranger stated.
I have a project with a file structure like this
main.py
input/
__init__.py
foo.py
Inside main I import Foo and create an instance of it with these lines:
main.py
from input.foo import Foo
foo = Foo()
Inside foo.py I need to use threading so I import it and create a thread in init.
foo.py
from threading import Thread
class Foo
def __init__(self):
newThread = Thread()
Later I decided it would be useful to have a thread in main.py so I also imported it there using from threading import Thread. Everything worked as it should up to this point. I thought it was redundant to import Thread twice during the execution so I removed the top line from foo.py and ran it again. This time I got NameError: global name 'Thread' is not defined when the program reached the line newThread = Thread().
Is there a way for me to do an import in main.py and not import it again in foo.py? It feels wrong to import the same class twice in one program.
Yes, you should place all the imports and dependencies at the top of all your modules, even if other modules have the same dependencies. Imports in python only exist within the scope of that module. Even though the import statement exists twice, python is smart enough to know that it's already been imported and won't have to actually re-import the module.
I have some code spread across multiple files that try to import from each other, as follows:
main.py:
from entity import Ent
entity.py:
from physics import Physics
class Ent:
...
physics.py:
from entity import Ent
class Physics:
...
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'm assume the error is due to importing entity twice - once in main.py and later in physics.py - but how can I work around the problem?
See also What happens when using mutual or circular (cyclic) imports in Python? for a general overview of what is allowed and what causes a problem WRT circular imports. See Why do circular imports seemingly work further up in the call stack but then raise an ImportError further down? for technical details on why and how the problem occurs.
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.
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.
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.
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.
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
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()
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.
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 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!
Try this solution: rename your working python script
You should not name your current python script with the name of some other module you import, since you will get that error.
Example:
you are working in medicaltorch.py
in that script, you have: from medicaltorch import X where medicaltorch is supposed to be a separate installed module
This will fail with the ImportError since 2 things refer to medicaltorch
So, just rename your working python script in 1.
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__'
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 🤦♂️
One way to track import error is step by step trying to run python on each of imported files to track down bad one.
you get something like:
python ./main.py
ImportError: cannot import name A
then you launch:
python ./modules/a.py
ImportError: cannot import name B
then you launch:
python ./modules/b.py
ImportError: cannot import name C (some NON-Existing module or some other error)
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.
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.
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.
I met this error too, but my case is less common, and it does throw this error too.
My case is that I encounter this error in jupyter notebook; I write from M import c where M is a python file and c is a class in M.py, the reason for the error is because c is just created a few minutes ago, but my jupyter notebook has been running for a long time, so I just need to restart the jupyter notebook and let it reload M.py.