How Can Circular Imports in Python Be Avoided and Resolved?

Circular Imports in Python: Pitfalls and Solutions

In the realm of Python programming, circular dependencies can arise when modules attempt to import each other. Understanding the behavior of such imports is crucial to avoid potential pitfalls.

When two modules, say foo and bar, mutually import each other using import foo and import bar, the imports will succeed seamlessly as both modules are fully loaded and can reference each other.

However, issues arise when using from imports, such as from foo import abc and from bar import xyz. In these scenarios, each module requires the other to be imported before it can proceed. This cyclical dependency leads to an import error.

To illustrate, consider the following code:

# foo.py
from bar import xyz

def abc():
    print(xyz.__name__)

# bar.py
from foo import abc

def xyz():
    print(abc.__name__)

This code will trigger an ImportError because foo requires bar to be imported before it can execute the from import statement, but bar also requires foo to be imported first.

To handle circular imports effectively, several solutions exist:

• Top of module; no from; Python 2 only:

  # foo.py
  import bar
  
  def abc():
    print(bar.xyz.__name__)
  
  # bar.py
  import foo
  
  def xyz():
    print(foo.abc.__name__)

• Top of module; from ok; relative ok; Python 3 only:

  # foo.py
  from . import bar
  
  def abc():
    print(bar.xyz.__name__)
  
  # bar.py
  from . import foo
  
  def xyz():
    print(abc.__name__)

• Top of module; no from; no relative:

  # foo.py
  import lib.bar
  
  def abc():
    print(lib.bar.xyz.__name__)
  
  # bar.py
  import lib.foo
  
  def xyz():
    print(lib.foo.abc.__name__)

• Bottom of module; import attribute, not module; from okay:

  # foo.py
  def abc():
    print(xyz.__name__)
  
  from .bar import xyz
  
  # bar.py
  def xyz():
    print(abc.__name__)
  
  from .foo import abc

• Top of function; from okay:

  # foo.py
  def abc():
    from . import bar
    print(bar.xyz.__name__)
  
  # bar.py
  def xyz():
    from . import foo
    print(foo.abc.__name__)

Star imports, however, are not discussed in the examples provided in the original article.

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