pydantic_type_coercion.py

from decimal import Decimal
from pydantic import BaseModel, ValidationError


class Expense(BaseModel):
    amount: Decimal


# coercion: each valid input becomes a Decimal
for v in [5, 5.50, "5.50", Decimal("5.50")]:
    got = Expense(amount=v).amount
    print(f"{v!r:18} -> {got!r:18} (type={type(got).__name__})")

print("-" * 55)

# rejection: anything that isn't a number
for v in ["abc", None, [5]]:
    try:
        Expense(amount=v)
        print(f"{v!r:18} -> accepted (unexpected)")
    except ValidationError as e:
        print(f"{v!r:18} -> rejected: {e.errors()[0]['msg']}")

"""
output:
5                  -> Decimal('5')       (type=Decimal)
5.5                -> Decimal('5.5')     (type=Decimal)
'5.50'             -> Decimal('5.50')    (type=Decimal)
Decimal('5.50')    -> Decimal('5.50')    (type=Decimal)
-------------------------------------------------------
'abc'              -> rejected: Input should be a valid decimal
None               -> rejected: Decimal input should be an integer, float, string or Decimal object
[5]                -> rejected: Decimal input should be an integer, float, string or Decimal object
"""