On 8/10/2013 2:00 PM, Xavi wrote:
Hello,
El 10/08/2013 18:40, Tim Chase escribió:
Generally, if you are using the "is" operator to compare against
anything other than None, you're doing it wrong. There are exceptions
to this, but it takes knowing the particulars.
Now I have one doubt, I use 'is' to compare basic types in python 3, for
example .-
v = []
if type(v) is list:
print('Is list...')
Because I think it is more clear and faster than .-
type(v) == [].__class__ ... or ... isinstance(v, list)
Is this correct?
It depends on the context. If one is writing a test for a function that
is defined as returning a list, such as the builtin function *sorted*,
then 'is list' would be correct.
When one knows the type, as in your toy snippet, 'is list' is nonsensical.
In a more typical situation, as when testing the argument to a function
in the body of a function, then 'isinstance(arg, list)' is almost
certainly more correct (but often still not best) as the function should
usually accept at least any list subclass instance.
def popslice(lis, start, stop=None, step=0):
if not isinstance(lis, list):
raise TypeError("Can only popslice a list")
if stop is None: # here is where is *should* be used
start, stop = 0, start
ret = lis[start:stop:step]
del lis[start:stop:step]
return ret
lis = list(range(10))
print(popslice(lis, 2, 9, 2), lis)
>>>
[2, 4, 6, 8] [0, 1, 3, 5, 7, 9]
However, why exclude a mutable sequence that support slices but is not
specifically a list?
def popslice(seq, start, stop=None, step=0):
if stop is None: # here is where is *should* be used
start, stop = 0, start
ret = seq[start:stop:step]
del seq[start:stop:step]
return ret
Bad inputs will raise TypeErrors.
TypeError: 'int' object is not subscriptable
TypeError: 'tuple' object doesn't support item deletion
It this is not good enough, wrap the body in
try:
...
except TypeError as e:
raise TypeError("your custom message here")
--
Terry Jan Reedy
--
http://mail.python.org/mailman/listinfo/python-list
