Writing two classes in separate files that have conversions to each other?

Question

Let's say I have two files, each with a class in it. int.py, which has a custom implementation of an integer class, and float.py, which has a custom implementation of a float class.

I want each class to have a conversion method to the other. For example:

class Integer:   
  def __init__(self, value):
    self.value = value

  def to_f():
    return Float(self.value)

and

class Float:   
  def __init__(self, value):
    self.value = value

  def to_i():
    return Integer(self.value)

How can I import the files into each other so that the constructors are available, without causing a circular dependency?

Put them both in the same file - you're about to run into circular import issues — inspectorG4dget, May 18 '17 at 17:39

Blender · Accepted Answer · 2017-05-18T17:58:18.197

You can import the class when the method is called:

class Float:   
  def __init__(self, value):
    self.value = value

  def to_i(self):
    from integer import Integer
    return Integer(self.value)

Imports are cached in sys.modules, so it won't have any performance penalty beyond the first call to Float.to_i.

An alternative is to import the module itself and look up the class:

import integer

class Float:   
  def __init__(self, value):
    self.value = value

  def to_i(self):
    return integer.Integer(self.value)

As long as there are no module-level circular dependencies (like subclassing Float in integer), you won't have any issues.

Yet another (potentially confusing) alternative is to import the classes after the modules:

class Float:   
  def __init__(self, value):
    self.value = value

  def to_i(self):
    return Integer(self.value)


from integer import Integer

By the time Float.to_i is called, Integer will be in scope. I only recall seeing the first two methods used in actual code.

score 0 · Answer 2 · answered May 18 '17 at 17:47

Just import the classes locally:

class Float:  
  def to_i():
    from .int import Integer  # not run at module loading time
    return Integer(self.value)

class Integer:   
  def to_f():
    from .float import Float
    return Float(self.value)

This prevents the imports from being run at module loading time and, thus, avoids circular imports.

Writing two classes in separate files that have conversions to each other?

2 Answers2