pythonpycharmstatic-analysispylintflake8

How can tools catch 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

  • 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.