Empty tuple type

[gpkc]

Bug

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

             pydantic version: 1.7.3
            pydantic compiled: True
                 install path: /home/guilherme/PycharmProjects/bulk-uploader/venv/lib/python3.8/site-packages/pydantic
               python version: 3.8.5 (default, Jul 28 2020, 12:59:40)  [GCC 9.3.0]
                     platform: Linux-5.4.0-65-generic-x86_64-with-glibc2.29
     optional deps. installed: ['typing-extensions']

from pydantic import BaseModel

class Example(BaseModel):
    example: Tuple[()]

Will result in:

Traceback (most recent call last):
  File "pydantic/validators.py", line 615, in pydantic.validators.find_validators
TypeError: issubclass() arg 1 must be a class

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...", line 12, in <module>
    class Example(BaseModel):
  File "pydantic/main.py", line 262, in pydantic.main.ModelMetaclass.__new__
  File "pydantic/fields.py", line 315, in pydantic.fields.ModelField.infer
  File "pydantic/fields.py", line 284, in pydantic.fields.ModelField.__init__
  File "pydantic/fields.py", line 356, in pydantic.fields.ModelField.prepare
  File "pydantic/fields.py", line 458, in pydantic.fields.ModelField._type_analysis
  File "pydantic/fields.py", line 516, in pydantic.fields.ModelField._create_sub_type
  File "pydantic/fields.py", line 284, in pydantic.fields.ModelField.__init__
  File "pydantic/fields.py", line 362, in pydantic.fields.ModelField.prepare
  File "pydantic/fields.py", line 538, in pydantic.fields.ModelField.populate_validators
  File "pydantic/validators.py", line 624, in find_validators
RuntimeError: error checking inheritance of () (type: tuple)

[mpkocher]

This looks like you might be mixing up types and instances by using example: Tuple[()] For example, List[dict()] doesn't really make sense from a type standpoint. The "empty" tuple might be better addressed by using a default_factory. This makes it consistent with other defaults, such as dict or set.

You can specify a variable length tuple with a single type as example: Tuple[int, ...]. It's possible to use Tuple[Any, ...] for a heterogeneous tuple, however, this isn't particularly useful from a type standpoint.

Here's an explicit example:

from typing import Tuple
from pydantic import BaseModel, Field

class T(BaseModel):
    example: Tuple[int, ...] = Field(default_factory=tuple)

def demo() -> int:

    a = T()
    b = T(example=(1,2))

    for x in (a, b):
        print(x)

    return 0


demo()

[PrettyWood]

@mpkocher the syntax may not be the one you expected but it's the one chosen in PEP 484

Tuple, used by listing the element types, for example Tuple[int, int, str]. The empty tuple can be typed as Tuple[()]. Arbitrary-length homogeneous tuples can be expressed using one type and ellipsis, for example Tuple[int, ...]. (The ... here are part of the syntax, a literal ellipsis.)

So if you want to enforce in pydantic an empty tuple, you don't want to set Tuple[Any, ...] but really Tuple[()].
@gpkc I opened a small PR to support this syntax. Thanks for reporting!

[mpkocher]

@PrettyWood Thanks for clarifying.