Unintentional trailing comma that creates a tuple

In Python, leaving a trailing comma like this is, of course, not a SyntaxError:

In [1]: x = 1 ,

In [2]: x
Out[2]: (1,)

In [3]: type(x)
Out[3]: tuple

But, at the same time, if the trailing comma was put accidentally, it may be difficult to catch this kind of a "problem", especially for Python newcomers.

I am thinking if we can catch this kind of a "problem" early, statically, with the help of PyCharm smart code quality control features; mypy, pylint or flake8 static code analysis tools.

Or, another idea would be to restrict/highlight creating one item tuples implicitly without parenthesis. Is it possible?

Solution 1:

pylintalready detects this as a problem (as of version 1.7).

For example, here's my tuple.py:

"""Module docstring to satisfy pylint"""

def main():
    """The main function"""
    thing = 1,
    print(type(thing))

if __name__ == "__main__":
    main()

$ pylint tuple.py
No config file found, using default configuration
************* Module tuple
R:  5, 0: Disallow trailing comma tuple (trailing-comma-tuple)

------------------------------------------------------------------
Your code has been rated at 8.00/10 (previous run: 8.00/10, +0.00)

$ pylint --help-msg trailing-comma-tuple
No config file found, using default configuration
:trailing-comma-tuple (R1707): *Disallow trailing comma tuple*
  In Python, a tuple is actually created by the comma symbol, not by the
  parentheses. Unfortunately, one can actually create a tuple by misplacing a
  trailing comma, which can lead to potential weird bugs in your code. You
  should always use parentheses explicitly for creating a tuple. This message
  belongs to the refactoring checker. It can't be emitted when using Python <
  3.0.

Unintentional trailing comma that creates a tuple

Solution 1:

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