Resolve circular import error in python

Question

Possible Duplicate:
Circular (or cyclic) imports in Python

I have class B that imports and creates instances of class A. Class A needs reference to B in its contructor and so includes B.

from a import A
class B:
  def __init__(self):
    self.a = A()

from b import B
class A:
  def __init__(self, ref):
    assert isinstance(ref, B)
    self.ref = ref

This doesn't work. The main file imports B and uses it... not. Something with the imports is wrong.

Error from file a ImportError: cannot import name B

Not an answer but it is probably best to have a better design which doesn't require this circular import. — jamylak, Apr 05 '12 at 10:40
Understood. But this seems like a basic problem. Class needs reference to upper level... — HWende, Apr 05 '12 at 10:41

score 20 · Accepted Answer · answered Apr 05 '12 at 10:43

20

Apart from "don't do that, you are painting yourself into a corner", you could also postpone the import of B until you need it. File a.py:

class A:
    def __init__(self, ref):
        from b import B
        assert isinstance(ref, B)
        self.ref = ref

Class B won't be imported until you instantiate class A, by which time the module already has been imported fully by module b.

You can also use a common base class and test for that.

answered Apr 05 '12 at 10:43

Martijn Pieters

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Thanks, I will go with that. I somewhere saw this idea but couldn't understand. Your additional explanation is excellent! – HWende Apr 05 '12 at 10:49

score 3 · Answer 2 · edited May 26 '12 at 12:38

3

Just import classes in __init__ method

class A:
   def __init__(self, ref):
      from b import B
      assert isinstance(ref, B)
      self.ref = ref

edited May 26 '12 at 12:38

Martijn Pieters

1,048,767
296
4,058
3,343

answered Apr 05 '12 at 10:43

Ildus

143
8

score 1 · Answer 3 · answered Apr 05 '12 at 11:26

The __init__ method executes when you create an instance of the class. In this case, you should get it to work by simply changing the import statements like this:

import a 
class B:
  def __init__(self):
    self.a = a.A(self)

import b
class A:
  def __init__(self, ref):
    assert isinstance(ref, b.B)
    self.ref = ref

Resolve circular import error in python

3 Answers3

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