(prepep) Preventing future issues with warnings as errors

Nineteendo · October 29, 2025, 2:31pm

Here’s a code block with all the syntax warnings (slightly edited):

PEP 765: Disallow return/break/continue that exit a finally block

x = [x + 0x1for x in [1]]  # SyntaxWarning: invalid decimal literal (ruff: nothing)
x = "\99"  # SyntaxWarning: "\9" is an invalid escape sequence (ruff: W605)
x = f"\{1}"  # SyntaxWarning: "\{" is an invalid escape sequence (ruff: W605)

def f():
    try:
        return 1 / 0
    finally:
        return 42  # SyntaxWarning: 'return' in a 'finally' block (ruff: B012)

x = 1 is 1  # SyntaxWarning: "is" with 'int' literal (ruff: F632)
assert (1, "msg")  # SyntaxWarning: assertion is always true, perhaps remove parentheses? (ruff: F631)
{}()  # SyntaxWarning: 'dict' object is not callable (ruff: nothing)
{1}[1]  # SyntaxWarning: 'set' object is not subscriptable (ruff: nothing)
"a"["b"]  # SyntaxWarning: str indices must be integers or slices, not str (ruff: RUF016)

So, what’s literally always incorrect here?

Literal divided by zero (currently not a warning):
```
1 / 0
```
Equivalent to:
```
raise ZeroDivisionError("division by zero")
```

Return that’s overwritten in a finally block (warning in a different place):

try:
    return 1 # <- warn here
finally:
    return 2 # <- not here

Equivalent to:

try:
    pass
finally:
    return 2

Assert with parentheses:
```
assert (x, "msg")
```
Equivalent to:
```
pass
```

Invalid operations on displays:

{}()
{1}[1]
"a"["b"]

Equivalent to:

raise TypeError("'dict' object is not callable")
raise TypeError("'set' object is not subscriptable")
raise TypeError("string indices must be integers, not 'str'")

What should display a warning anyway?

Ambiguous syntax:

x = [y + 0x1for y in [1]]

Equivalent to:

x = [y + 0x1f or y in [1]]

Strict equality (only works with small numbers):
```
x = y is 1
```
Equivalent to:
```
x = y == 1 and isinstance(y, int)
```
Single backslashes (only works with invalid escape sequences):
```
x = "\99"
x = f"\{1}"
```
Equivalent to:
```
x = "\\99"
x = f"\\{1}"
```

And what’s more controversial:

Supressing exceptions in finally:

try:
    input() # Ctrl+C
finally:
    return 1

Equivalent to:

try:
    input() # Ctrl+C
except:
    pass
return 1