I was very impressed with the collective thought process shown in the bug thread. It was quite constructive and I think demonstrative of why python has been such a successful project.
It used to be quite bad, but it's getting better in a good rate. Of course they have to keep bad stuff around for backwards compatibility, but unlike say Java they wasn't afraid to break too if it was for something smaller.
>>> import warnings
>>> warnings.simplefilter('always')
>>> 'x'*np.float64(3.5)
__main__:1: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
'xxx'
It would be a warning from numpy, not Python. That comes from numpy/core/src/multiarray/conversion_utils.c and was added 2013-04-13 , which would be for NumPy 1.8, I believe.
I use R predominantly, so forgive the lack of adequate Python-ese.
Is this behavior because numpy overloads the multiplication operation with a string as string repetition and then implicitly casts the float64 down to an integer of 3? I'm curious why this behavior manifests. When I get a chance I'll test 'xyz'*np.float(3.5)
I thought that at first too, but even then the exception seems spurious. It would be more inline with existing Python behavior to evaluate it as undefined/indeterminate assignment order (though this would be pretty useless).
>>> a, b, c, d = {1, 2, 3, 'a'}
>>> a
'a'
Regardless, it shouldn't be surprising when writing insane code, that the language starts acting insane. Python as a whole is pretty decent when it comes to handling syntactic edge cases.
The tricky part is that the [a,b] and (a,b) in your original examples are not list/tuple literals, but a target_list according to Python's grammar [0], which can be optionally enclosed in parentheses or brackets. To be honest, I didn't know the latter were allowed, even though I've been using Python for years. IMHO, they should have just allowed parentheses. It's like the parentheses in function calls--not really a tuple, but syntactic grouping. Why have more that one way to do the same thing?
Only in Python 2, since that is a backward-compatibility feature. (“False” and “True” used to be variables containing 0 and 1 before Python had true booleans, many ages ago.) Python 3 makes “True” and “False” be constant values, like “1” or “2”, like they should be.
Yes. There are codes written with backwards compatibility to versions of Python pre-2.3, which did not have True/False. They typically looked like this:
Couldn't that be kept compatible by allowing tautological assignments to True/False (True = (1==1), that sort of thing) but throwing an error on all other assignments?
Also, I don't see how that wouldn't be compatible with forbidding assignments to True/False. It'd never execute the except block.
Yes, that specific case is possible with some sort of AST rewriting. But it's only one of several ways to introduce a True into the module or local namespace. Another might be:
from _compat import True, False, next, enumerate
This is allowed under Python 2, but not under Python 3. As you suggest, it could be made possible to allow an import like this so long as the actual imported value is indeed the True or False singleton. But it's a lot of work with little gain.
While on the other hand, even in Python 2 it was a SyntaxError to say "None = None" or to import None. Extending that check to include True and False is much easier, and consistent with existing use.
What bothers me here is that when the interpreter says "False", it clearly means the opposite of (the new) False. So by reassigning True and False, I've actually caused inconsistent behavior in python, not just really-confusingly-named behavior. Suddenly False sometimes means one thing and sometimes means something else.
There's no inconsistency, the "False" printed in your console is just the repr of the object, you can give that repr to your own object if you have nothing better to do with your life:
A repr is just a representation in development context, nothing more and nothing less, there is no requirement that it be sensible or of any use, though that's certainly recommended (contrary to __str__/__unicode__ which should be silly):
>>> B()
A gray parrot
The only "inconsistency" you've created is that False in the console's local namespace does not match __builtins__.False or the interpreter's internal False object. You can still access the former by the way:
the interpreter still does not care though, you've just broken any Python code relying on the builtin (you can actually alter the "true" False object via ctypes)
