I keep on having an ImportError in a complex project. I've distilled it to the bare minimum that still gives the error.
A wizard has containers with green and brown potions. These can be added together, resulting in new potions that are also either green or brown.
We have a Potion ABC, which gets its __add__, __neg__ and __mul__ from the PotionArithmatic mixin. Potion has 2 subclasses: GreenPotion and BrownPotion.
In one file, it looks like this:
onefile.py:
from abc import ABC, abstractmethod
def add_potion_instances(potion1, potion2): # some 'outsourced' arithmatic
return BrownPotion(potion1.volume + potion2.volume)
class PotionArithmatic:
def __add__(self, other):
# Adding potions always returns a brown potion.
if isinstance(other, base.Potion):
return add_potion_instances(self, other)
return BrownPotion(self.volume + other)
def __mul__(self, other):
# Multiplying a potion with a number scales it.
if isinstance(other, Potion):
raise TypeError("Cannot multiply Potions")
return self.__class__(self.volume * other)
def __neg__(self):
# Negating a potion changes its color but not its volume.
if isinstance(self, GreenPotion):
return BrownPotion(self.volume)
else: # isinstance(self, BrownPotion):
return GreenPotion(self.volume)
# (... and many more)
class Potion(ABC, PotionArithmatic):
def __init__(self, volume: float):
self.volume = volume
__repr__ = lambda self: f"{self.__class__.__name__} with volume of {self.volume} l."
@property
@abstractmethod
def color(self) -> str:
...
class GreenPotion(Potion):
color = "green"
class BrownPotion(Potion):
color = "brown"
if __name__ == "__main__":
b1 = GreenPotion(5)
b2 = BrownPotion(111)
b3 = b1 + b2
assert b3.volume == 116
assert type(b3) is BrownPotion
b4 = b1 * 3
assert b4.volume == 15
assert type(b4) is GreenPotion
b5 = b2 * 3
assert b5.volume == 333
assert type(b5) is BrownPotion
b6 = -b1
assert b6.volume == 5
assert type(b6) is BrownPotion
This works.
Each part is put in its own file inside the folder potions, like so:
usage.py
potions
| arithmatic.py
| base.py
| green.py
| brown.py
| __init__.py
potions/arithmatic.py:
from . import base, brown, green
def add_potion_instances(potion1, potion2):
return brown.BrownPotion(potion1.volume + potion2.volume)
class PotionArithmatic:
def __add__(self, other):
# Adding potions always returns a brown potion.
if isinstance(other, base.Potion):
return add_potion_instances(self, other)
return brown.BrownPotion(self.volume + other)
def __mul__(self, other):
# Multiplying a potion with a number scales it.
if isinstance(other, base.Potion):
raise TypeError("Cannot multiply Potions")
return self.__class__(self.volume * other)
def __neg__(self):
# Negating a potion changes its color but not its volume.
if isinstance(self, green.GreenPotion):
return brown.BrownPotion(self.volume)
else: # isinstance(self, BrownPotion):
return green.GreenPotion(self.volume)
potions/base.py:
from abc import ABC, abstractmethod
from .arithmatic import PotionArithmatic
class Potion(ABC, PotionArithmatic):
def __init__(self, volume: float):
self.volume = volume
__repr__ = lambda self: f"{self.__class__.__name__} with volume of {self.volume} l."
@property
@abstractmethod
def color(self) -> str:
...
potions/green.py:
from .base import Potion
class GreenPotion(Potion):
color = "green"
potions/brown.py:
from .base import Potion
class BrownPotion(Potion):
color = "brown"
potions/__init__.py:
from .base import Potion
from .brown import GreenPotion
from .brown import BrownPotion
usage.py:
from potions import GreenPotion, BrownPotion
b1 = GreenPotion(5)
b2 = BrownPotion(111)
b3 = b1 + b2
assert b3.volume == 116
assert type(b3) is BrownPotion
b4 = b1 * 3
assert b4.volume == 15
assert type(b4) is GreenPotion
b5 = b2 * 3
assert b5.volume == 333
assert type(b5) is BrownPotion
b6 = -b1
assert b6.volume == 5
assert type(b6) is BrownPotion
Running usage.py gives the following ImportError:
ImportError Traceback (most recent call last)
usage.py in <module>
----> 1 from potions import GreenPotion, BrownPotion
2
3 b1 = GreenPotion(5)
4 b2 = BrownPotion(111)
5
potions\__init__.py in <module>
----> 1 from .green import GreenPotion
2 from .brown import BrownPotion
potions\brown.py in <module>
----> 1 from .base import Potion
2
3 class GreenPotion(Potion):
4 color = "green"
potions\base.py in <module>
1 from abc import ABC, abstractmethod
2
----> 3 from .arithmatic import PotionArithmatic
4
potions\arithmatic.py in <module>
----> 1 from . import base, brown, green
2
3 class PotionArithmatic:
4 def __add__(self, other):
potions\green.py in <module>
----> 1 from .base import Potion
2
3 class GreenPotion(Potion):
4 color = "green"
ImportError: cannot import name 'Potion' from partially initialized module 'potions.base' (most likely due to a circular import) (potions\base.py)
Potion is a subclass of the mixin PotionArithmatic, the import of PotionArithmatic in base.py cannot be changed.GreenPotion and BrownPotion are subclasses of Potion, the import of Potion in green.py and brown.py cannot be changed.arithmatic.py. This is where the change must be made.I've looked for hours and hours into this type of problem.
The usual solution is to not import the classes Potion, GreenPotion, and BrownPotion into the file arithmatic.py, but rather import the files in their entirety, and access the classes with base.Potion, green.GreenPotion, brown.BrownPotion. This I have already done in the code above, and does not solve my problem.
A possible solution is to move the imports into the functions that need them, like so:
arithmatic.py:
def add_potion_instances(potion1, potion2):
from . import base, brown, green # <-- added imports here
return brown.BrownPotion(potion1.volume + potion2.volume)
class PotionArithmatic:
def __add__(self, other):
from . import base, brown, green # <-- added imports here
# Adding potions always returns a brown potion.
if isinstance(other, base.Potion):
return add_potion_instances(self, other)
return brown.BrownPotion(self.volume + other)
def __mul__(self, other):
from . import base, brown, green # <-- added imports here
# Multiplying a potion with a number scales it.
if isinstance(other, base.Potion):
raise TypeError("Cannot multiply Potions")
return self.__class__(self.volume * other)
def __neg__(self):
from . import base, brown, green # <-- added imports here
# Negating a potion changes its color but not its volume.
if isinstance(self, green.GreenPotion):
return brown.BrownPotion(self.volume)
else: # isinstance(self, BrownPotion):
return green.GreenPotion(self.volume)
Though this works, you can imagine this results in many additional lines if the file contains many more methods for the mixin class, esp. if these in turn call functions on the module's top level.
Many thanks!
you should not use on the mixin/inheritance architecture if the mixin returns an instance of the class (or of one of its descendants). In that case, the methods should be appended to the class object itself.
I thought of 2 (very similar) ways to get it to work. None is ideal, but they both seem to resolve the problem, by no longer relying on inheritance for the mixin.
In both, the potions/base.py file is changed to the following:
potions/base.py:
from abc import ABC, abstractmethod
class Potion(ABC): # <-- mixin is gone
# (nothing changed here)
from . import arithmatic # <-- moved to the end
arithmatic.append_methods() # <-- explicitly 'do the thing'
What we do with potions/arithmatic.py depends on the solution.
This solution I like the best. In arithmatic.py, we can keep the original PotionArithmatic class. We just add a list of relevant dunder methods it, and the append_methods() function to do the appending.
potions/arithmatic.py:
from . import base, brown, green
def add_potion_instances(potion1, potion2):
# (nothing changed here)
def PotionArithmatic:
ATTRIBUTES = ["__add__", "__mul__", "__neg__"] # <-- this is new
# (nothing else changed here)
def append_methods(): # <-- this is new as well
for attr in PotionArithmatic.ATTRIBUTES:
setattr(base.Potion, attr, getattr(PotionArithmatic, attr))
Alternatively, we can get rid of the PotionArithmatic class alltogether, and just append the methods directly to the Potion class object:
potions/arithmatic.py:
from . import base, brown, green
def _add_potion_instances(potion1, potion2):
return brown.BrownPotion(potion1.volume + potion2.volume)
def _ext_add(self, other):
# Adding potions always returns a brown potion.
if isinstance(other, base.Potion):
return _add_potion_instances(self, other)
return brown.BrownPotion(self.volume + other)
def _ext_mul(self, other):
# Multiplying a potion with a number scales it.
if isinstance(other, base.Potion):
raise TypeError("Cannot multiply Potions")
return self.__class__(self.volume * other)
def _ext_neg(self):
# Negating a potion changes its color but not its volume.
if isinstance(self, green.GreenPotion):
return brown.BrownPotion(self.volume)
else: # isinstance(self, BrownPotion):
return green.GreenPotion(self.volume)
def append_methods():
base.Potion.__add__ = _ext_add
base.Potion.__mul__ = _ext_mul
base.Potion.__neg__ = _ext_neg
Both solutions work, but be aware that
(a) they introduce more coupling and necessitate moving imports to the end of base.py, and
(b) the IDE will no longer know about these methods when writing code, as they are added at run-time.