Abstract

This PEP proposes allowing parentheses in the two-argument form of assert. The interpreter will reinterpret assert (expr, msg) as assert expr, msg, eliminating the common pitfall where such code was previously treated as asserting a two-element tuple, which is always truthy.

Motivation

It is a common user mistake when using the form of the assert statement that includes the error message to surround it with parentheses [1] [2]. This is because many beginners assume assert is a function. The prominent unittest methods, particularly assertTrue(), also require parentheses around the assertion and message.

Unfortunately, this mistake passes undetected as the assert will always pass [6], because it is interpreted as an assert statement where the expression is a two-tuple, which always has truth-y value. The mistake also often occurs when extending the test or description beyond a single line, as parentheses are a natural way to do that.

This is so common that a SyntaxWarning is emitted by the compiler since 3.10 and several code linters [3] [4].

Additionally, some other statements in the language allow parenthesized forms in one way or another, for example, import statements (from x import (a,b,c)) or del statements (del (a,b,c)).

Allowing parentheses not only will remove the pitfall but also will allow users and auto-formatters to format long assert statements over multiple lines in what the authors of this document believe will be a more natural way. Although it is possible to currently format long assert statements over multiple lines with backslashes (as is recommended by PEP 8) or parentheses and a comma:

assert (
  very very long
  test
), (
  "very very long "
  "error message"
)

the authors of this document believe the proposed parenthesized form is more clear and intuitive, along with being more consistent with the formatting of other grammar constructs:

assert (
  very very long
  test,

  "very very long "
  "message"
)

Rationale

Due to backwards compatibility concerns (see section below), to inform users of the new change of how what was previously a two element tuple is parsed, a SyntaxWarning with a message like "new assertion syntax, will assert first element of tuple" will be raised till Python 3.17. For example, when using the new syntax:

>>> assert ('Petr' == 'Pablo', "That doesn't look right!")
<python-input-0>:0: SyntaxWarning: new assertion syntax, will assert first element of tuple
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    assert ('Petr' == 'Pablo', "That doesn't look right!")
            ^^^^^^^^^^^^^^^^^
AssertionError: That doesn't look right!

Note that improving syntax warnings in general is out of the scope of this PEP.

Specification

The formal grammar of the assert statement will change to [8]:

| 'assert' '(' expression ',' expression [','] ')' &(NEWLINE | ';')
| 'assert' a=expression [',' expression ]

Where the first line is the new form of the assert statement that allows parentheses and will raise a SyntaxWarning till 3.17. The lookahead is needed to prevent the parser from eagerly capturing the tuple as the full statement, so statements like assert (a, b) <= c, "something" are still parsed correctly.

Implementation Notes

This change can be implemented in the parser or in the compiler. The specification that a SyntaxWarning be raised informing users of the new syntax complicates the implementation, as warnings should be raised during compilation.

The authors believe that an ideal implementation would be in the parser [8], resulting in assert (x,y) having the same AST as assert x,y. This necessitates a two-step implementation plan, with a necessary temporary compromise.

>>> print(ast.dump(ast.parse('assert(True, "Error message")'), indent=4))
Module(
    body=[
        Assert(
            test=Constant(value=True),
            msg=Constant(value='Error message'),
            paren_syntax=1)])

Backwards Compatibility

The change is not technically backwards compatible. Whether implemented initially in the parser or the compiler, assert (x,y), which is currently interpreted as an assert statement with a 2-tuple as the subject and is always truth-y, will be interpreted as assert x,y.

On the other hand, assert statements of this kind always pass, so they are effectively not doing anything in user code. The authors of this document think that this backwards incompatibility nature is beneficial, as it will highlight these cases in user code while before they will have passed unnoticed. This case has already raised a SyntaxWarning since Python 3.10, so there has been a deprecation period of over 5 years. The continued raising of a SyntaxWarning should mitigate surprises.

The change will also result in changes to the AST of assert (x,y), which currently is:

Module(
    body=[
        Assert(
            test=Tuple(
                elts=[
                    Name(id='x', ctx=Load()),
                    Name(id='y', ctx=Load())],
                ctx=Load()))],
    type_ignores=[])

the final implementation, in Python 3.18, will result in the following AST:

Module(
    body=[
        Assert(
            test=Name(id='x', ctx=Load()),
            msg=Name(id='y', ctx=Load()))],
    type_ignores=[])

The problem with this is that the AST of the first form will technically be “incorrect” as we already have a specialized form for the AST of an assert statement with a test and a message (the second one). Implementing initially in the compiler will delay this change, alleviating backwards compatibility concerns, as tools will have more time to adjust.

How to Teach This

The new form of the assert statement will be documented as part of the language standard.

When teaching the form with error message of the assert statement to users, now it can be noted that adding parentheses also work as expected, which allows to break the statement over multiple lines.

Reference Implementation

A reference implementation in the parser can be found in this branch and reference implementation in the compiler can be found in this branch.

Rejected Ideas

assert condition else "message"
assert (condition else "message")

Security Implications

There are no security implications for this change.

Acknowledgements

This change was originally discussed and proposed in python/cpython#90325.

Many thanks to Petr Viktorin for his help during the drafting process of this PEP.

Footnotes

Copyright

This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.