On Fri, Feb 10, 2017 at 7:59 PM, boB Stepp <robertvst...@gmail.com> wrote: > I have been following the thread "int vs. float" > (https://mail.python.org/pipermail/python-list/2017-February/719287.html) > on the main list. A search for the OP on the Tutor archive came up > negative, so I am hoping he is not following Tutor tonight (Or anytime > prior to the due date for his homework!). The central part of > adam14711993's question is: > > "What I cannot figure out is how to write it so that if my user input > is, for example, 1.5, the program will result with: Sorry, you can > only order whole packages. > > "I understand that because I am starting out by assigning my > number_purchases_str to be an int, when the user enters a float that > is a conflict and will crash." > > He cannot figure out how to reliably tell if the user's input is an > integer, float or neither.
Back in the main Python list thread, Marko Rauhamaa suggested (https://mail.python.org/pipermail/python-list/2017-February/719322.html): " ... Haven't been following the discussion, but this should be simply: ast.literal_eval("...") ... " This looks like it may do the trick quite concisely: py3: import ast py3: ast.literal_eval('5') 5 py3: ast.literal_eval('5.0') 5.0 py3: type(ast.literal_eval('5')) <class 'int'> py3: type(ast.literal_eval('5.0')) <class 'float'> It appears to reliably parse string input as to whether it is an integer or float. However, if the input is a non-numerical string: py3: ast.literal_eval('a') Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\Program Files\Python35\lib\ast.py", line 84, in literal_eval return _convert(node_or_string) File "C:\Program Files\Python35\lib\ast.py", line 83, in _convert raise ValueError('malformed node or string: ' + repr(node)) ValueError: malformed node or string: <_ast.Name object at 0x0000000002C9D860> But that seems okay, as in the OP's original problem spec, he just needed a way to determine integer, float or other non-acceptable input. This seems to do this quite directly. I am not familiar with "abstract syntax trees" (Python has so much very interesting stuff!). I am concerned about this paragraph at the beginning of the docs on it (https://docs.python.org/3/library/ast.html?highlight=ast.literal_eval#module-ast): "The ast module helps Python applications to process trees of the Python abstract syntax grammar. The abstract syntax itself might change with each Python release; this module helps to find out programmatically what the current grammar looks like." That statement about potentially changing with each Python release bothers me. Should I be bothered? The link to ast.literal_eval(): https://docs.python.org/3/library/ast.html?highlight=ast.literal_eval#ast.literal_eval What are the gotchas or other negatives from this approach? boB _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
