R. David Murray added the comment: Oops, sorry, I missed the fact that you were pointing out a doc issue. ---------- resolution: out of date -> stage: resolved -> needs patch status: closed -> open _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25299> _______________________________________

A. Skrobov added the comment: Thank you for confirming that the mismatch between the documentation and the behaviour is preserved in Python 3! Adding it to the list of affected versions. ---------- versions: +Python 3.5 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25299> _______________________________________

paul j3 added the comment: A related issue is http://bugs.python.org/issue24754 argparse add_argument with action="store_true", type=bool should not crash In this case the user specified a parameter that the 'store_true' subclass did not accept. In both cases, 'parser.add_argument' does minimal error checking (or catching), so the user ends up seeing the '__init__' argument error. This line in the 'store_const' documentation is clearly wrong. There's no default. (Note that the const keyword argument defaults to the rather unhelpful None.) Possible fixes to the bigger issue: - major addition to the documentation, documenting allowable subclass parameters (but the subclasses aren't part of the API). - major addition to add_argument that filters parameters before passing them to the subclasses. But can that be done without replicating information that is implicit in the subclass __init__? Can we use advanced inspection? What about user defined Action classes? - minor addition to add_argument that catches the __init__ parameter errors and adds a mollifing explanation. - somehow make Action (and its subclasses) responsible for a nice error message. But how do you override the normal behavior of Python regarding function parameters? ---------- nosy: +paul.j3 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25299> _______________________________________

paul j3 added the comment: A fix that I am exploring would wrap the Action instantiation call in add_argument with a try/except block That is replace: def add_argument(...) .... action = action_class(**kwargs) ... with .... try: action = action_class(**kwargs) except TypeError: msg = str(_sys.exc_info()[1]) msg = msg.replace('__init__','add_argument') msg = [msg] msg.append('Wrong argument(s) for Action subclass: %s'%action_class.__name__) # action_class is now a class, rather than the input string msg.append('kwargs: %s'%kwargs) sig = getattr(action_class, 'action_sig',None) if sig is not None: msg = sig(msg) raise ValueError(msg) .... This collects information on the error, the action class and arguments, and passes them to a class method of the Action. That customizes the message, and returns it for reraising. In 'class Action' I define: @classmethod def action_sig(cls, msg=[]): # return the signature # subclasses may return custom version import inspect try: name = cls.__name__ sig = inspect.signature(cls.__init__) sig = str(sig) except AttributeError: spec = inspect.getfullargspec(cls.__init__) sig = inspect.formatargspec(*spec) # remove self, option_strings, dest dstr='dest,' ind = sig.find(dstr) if ind>=0: sig = '(...'+sig[(ind+len(dstr)+1):] sig = 'class args: %s'%sig msg.append(sig) return '\n'.join(msg) This adds inspect.signature information from the subclass __init__ method to the message from add_argument. Subclasses (including the user defined ones) could customize this. So a missing 'const' error would display as: ValueError: add_argument() missing 1 required positional argument: 'const' Wrong argument(s) for Action subclass: _StoreConstAction kwargs: {'option_strings': ['--bar'], 'dest': 'bar'} class args: (...const, default=None, required=False, help=None, metavar=None) This may be overkill, but it gives an idea of the information that we can add to the original TypeError. Done right this action_sig() could also serve as a usage function for an Action class. ---------- keywords: +3.5regression _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25299> _______________________________________

Joannah Nanjekye <nanjekyejoannah@gmail.com> added the comment: paul.j3, The fix looks promising. Do you want to open a PR with this fix? ---------- nosy: +nanjekyejoannah _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue25299> _______________________________________

Joannah Nanjekye <nanjekyejoannah@gmail.com> added the comment:

this argument defaults to "None"

Am leaning to the fact that this works as required. The documentation does not just say defaults to none. It says: With the 'store_const' and 'append_const' actions, the const keyword argument must be given. For other actions, it defaults to None. In the scenario, you have given, you are using 'store_const' so the const keyword argument must be given. This should be closed If you agree with what I have just stated. ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue25299> _______________________________________

Joannah Nanjekye <nanjekyejoannah@gmail.com> added the comment: Since both the docs and implementation now match, I suggest we close this and open a new issue suggesting to change the implementation back to make const=None when action='store_const'. ---------- stage: needs patch -> resolved _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue25299> _______________________________________

Change by STINNER Victor <vstinner@redhat.com>: ---------- title: TypeError: __init__() takes at least 4 arguments (4 given) -> argparse: TypeError: __init__() takes at least 4 arguments (4 given) _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue25299> _______________________________________