Generate a hash function when `allow_mutation` is `False`

[rhuille]

Feature Request

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.6.1
            pydantic compiled: False
                 install path: /home/raphael/rhuille/pydantic/pydantic
               python version: 3.8.5 (default, Aug 18 2020, 10:43:36)  [GCC 7.5.0]
                     platform: Linux-5.0.0-1067-oem-osp1-x86_64-with-glibc2.27
     optional deps. installed: ['typing-extensions', 'email-validator', 'devtools']

Hi there,

I need the model to be hashable when I set allow_mutation to False. I could write myself a __hash__ function, but I think it could be nice if pydantic generated it by default in the class BaseModel.

This would be the same behavior of the built-in dataclass which generate a hash function if the object is immutable (parameter frozen is True) and the __eq__ function exists.

Quoting the documentation https://docs.python.org/3/library/dataclasses.html :

If eq and frozen are both true, by default dataclass() will generate a __hash__() method for you.

We could generate a default __hash__ function for BaseModel if allow_mutation is False, what do you think ?

Examples

For example, now we have:

from pydantic import BaseModel

class A(BaseModel):
    x: int
    
    class Config:
        allow_mutation = False

a = A(x=1)
d = {a: 2}

>>> TypeError: unhashable type: 'A'

After, we would have:

from pydantic import BaseModel

class A(BaseModel):
    x: int
    
    class Config:
        allow_mutation = False

a = A(x=1)
d = {a: 2}
d[a]
>>> 2

(Note that if allow_mutation = True, the hash function would be None and the behavior would not be changed)

[jolynch]

@rhuille @samuelcolvin I believe that the introduction of explicitly setting __hash__ to None for non frozen BaseModels is a backwards incompatible breaking change for models which inherit from a common BaseModel implementation that defines __hash__ and __eq__.

Minimal reproduction:

import pydantic                                                                 
from pydantic import BaseModel                                                  
                                                                                
print(f"Version: {pydantic.version.VERSION}")                                   
                                                                                
class HashableModel(BaseModel):                                                 
    x: int                                                                      
                                                                                
    class Config:                                                               
        allow_mutation = False                                                  
                                                                                
    def __hash__(self):                                                         
        return hash((type(self),) + tuple(self.__dict__.values()))              
                                                                                
    def __eq__(self, other):                                                    
        return self.__hash__() == other.__hash__()                              
                                                                                
class ChildModel(HashableModel):                                                
    y: int                                                                      
                                                                                
# This is fine                                                                  
test = {HashableModel(x=4): HashableModel(x=2)}                                 
print(test)                                                                     
                                                                                
# This will fail on 1.8                                                                                                                                                                                                                             
test = {ChildModel(x=4, y=2): ChildModel(x=2, y=2)}                             
print(test)

On 1.7.3:

Version: 1.7.3
{HashableModel(x=4): HashableModel(x=2)}
{ChildModel(x=4, y=2): ChildModel(x=2, y=2)}

On 1.8:

Version: 1.8
{HashableModel(x=4): HashableModel(x=2)}
Traceback (most recent call last):
  File "repro.py", line 26, in <module>
    test = {ChildModel(x=4, y=2): ChildModel(x=2, y=2)}
TypeError: unhashable type: 'ChildModel'

A temporary workaround that is both forwards and backwards compatible is to set frozen explicitly to True on the superclass:

class Config:                                                               
    allow_mutation = False  
    frozen = True

Looking at the patch I'm not sure if there is a simple way to fix this, but I imagine this might hit some folks who have previously defined a common ImmutableBaseModel and are using that instead of BaseModel various places.

[samuelcolvin]

Is this fixed by #2423?

[jolynch]

Yes it is fixed by that patch, verified with:

$ python3 -m venv venv_test
$ venv_test/bin/pip install wheel
$ venv_test/bin/pip install git+https://github.com/samuelcolvin/pydantic.git@f9fe4aa086e7458f3c3339c408b3215f34e6ca18
$ venv_test/bin/python repro.py
Version: 1.8
{HashableModel(x=4): HashableModel(x=2)}
{ChildModel(x=4, y=2): ChildModel(x=2, y=2)}

Thanks and apologies for not seeing the new patch!

[ekampf]

@samuelcolvin any chance you can push a 1.8.1 with that fix?