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:
pylint
already 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.