Mypy allows to compare NewTypes created from Decimal

[kulych]

Bug Report

Mypy does not produce an error when comparing NewTypes based on Decimal.

To Reproduce

from decimal import Decimal
from typing import NewType

# Comparing Decimal NewTypes is OK 
FirstDecimal = NewType("FirstDecimal", Decimal)
SecondDecimal = NewType("SecondDecimal", Decimal)

FirstDecimal(Decimal("0.1")) == SecondDecimal(Decimal("0.1"))  # ! mypy ok

# But comparing other NewTypes is not
FirstInt = NewType("FirstInt", int)
SecondInt = NewType("SecondInt", int)

FirstInt(1) == SecondInt(3)  # mypy error

FirstString = NewType("FirstString", str)
SecondString = NewType("SecondString", str)

FirstString("bam") == SecondString("boo")  # mypy error

Expected Behavior

Mypy produces Non-overlapping equality check for all three comparisons.

Actual Behavior

Mypy produces Non-overlapping equality check only for int and str NewTypes.

Your Environment

• Mypy version used: 1.1.1
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
strict = true
show_error_context = true
show_error_codes = true

• Python version used: 3.10.8

[erictraut]

I'm surprised that mypy would generate comparison-overlap errors for any of these cases. As I understand it, comparison-overlap is intended to detect cases where a comparison will always produce True or False because the types of the operands are not comparable. This is how the feature is documented.

In the three cases above, the operand types are comparable, and they can produce either True or False depending on the values.

print(FirstDecimal(Decimal("0.1")) == SecondDecimal(Decimal("0.2"))) # False
print(FirstDecimal(Decimal("0.1")) == SecondDecimal(Decimal("0.1"))) # True

print(FirstString("bam") == SecondString("boo")) # False
print(FirstString("boo") == SecondString("boo")) # True

So, I think mypy shouldn't produce comparison-overlap errors for any of these cases.

[kulych]

Just to improve my provided example:

from decimal import Decimal
from typing import NewType

# Comparing Decimal NewTypes is OK 
FirstDecimal = NewType("FirstDecimal", Decimal)
SecondDecimal = NewType("SecondDecimal", Decimal)

result_decimal_false = FirstDecimal(Decimal("0.6")) == SecondDecimal(Decimal("0.1"))  # ! mypy ok
result_decimal_true = FirstDecimal(Decimal("0.1")) == SecondDecimal(Decimal("0.1"))  # ! mypy ok
print("Decimal false:", result_decimal_false)
print("Decimal true:", result_decimal_true)
print()

# But comparing other NewTypes is not
FirstInt = NewType("FirstInt", int)
SecondInt = NewType("SecondInt", int)

result_int_false = FirstInt(1) == SecondInt(3)  # mypy error
result_int_true = FirstInt(1) == SecondInt(1)  # mypy error
print("Float false:", result_int_false)
print("Float true:", result_int_true)
print()

FirstString = NewType("FirstString", str)
SecondString = NewType("SecondString", str)

result_string_false = FirstString("bam") == SecondString("boo")  # mypy error
result_string_true = FirstString("bam") == SecondString("bam")  # mypy error

print("String false:", result_string_false)
print("String true:", result_string_true)

Produces (as @erictraut correctly noted)

Decimal false: False
Decimal true: True

Float false: False
Float true: True

String false: False
String true: True

And mypy output is:

(venv) [~/tmp]$ mypy --strict test_mypy.py
test_mypy.py:18: error: Non-overlapping equality check (left operand type: "FirstInt", right operand type: "SecondInt")  [comparison-overlap]
test_mypy.py:19: error: Non-overlapping equality check (left operand type: "FirstInt", right operand type: "SecondInt")  [comparison-overlap]
test_mypy.py:27: error: Non-overlapping equality check (left operand type: "FirstString", right operand type: "SecondString")  [comparison-overlap]
test_mypy.py:28: error: Non-overlapping equality check (left operand type: "FirstString", right operand type: "SecondString")  [comparison-overlap]
Found 4 errors in 1 file (checked 1 source file)

I am in no means a mypy expert so please take this issues only as an inconsistency report.

[ikonst]

comparison-overlap, as defined, indeed seems incorrect as the thing that flags NewType violations. We should either rewrite the document or make a new error code. The checking itself, however, seems valuable, as the point of having a NewType is to make the checker yell at you for, say, interchanging miles and meters.