On 7/12/19 11:39 AM, Alan Gauld via Tutor wrote:
> On 12/07/2019 15:24, Gursimran Maken wrote:
>
>> Can someone please explain me the reason for below output.
>
> You've been bitten by one of the most common gotchas in Python :-)
>
>> def fun(n,li = []):
>> a = list(range(5))
>> li.append(a)
>> print(li)
>>
>> fun(4)
>> fun(5,[7,8,9])
>> fun(4,[7,8,9])
>> fun(5) # reason for output (why am I getting to values in this output.)
>
> When you define a default value in Python it creates the default value
> at the time you define the function. It then uses that value each time a
> default is needed. in the case of a list that means Python creates an
> empty list and stores it for use as the default.
It may help in seeing why this happens to be aware that a def: statement
is an executable statement like any other, which is executed at the time
it is reached in the file. Running it generates a function object, a
reference to it being attached to the name of the function. Conceptually,
def foo():
is like
foo = FunctionConstructor()
foo ends up referring to an object that is marked as callable so you can
then later call it as foo(). So it makes some sense that some things
have to happen at object construction time, like setting up the things
that are to be used as defaults.
> When you first call the function with the default Python adds values to
> the defaiult list.
>
> Second time you call the function using the default Python adds (more)
> values to (the same) default list.
FWIW, Python does the same no matter the type of the default argument,
but it only causes the trap we poor programmers fall into if the type is
one that can be modified ("mutable"). If you have fun(n, a=5) or fun(n,
s="stringystuff") those are unchangeable and we don't get this little
surprise.
By the way, checker tools (and IDEs/editors with embedded checking
capabilities) will warn about this, which is a hint on using good tools.
pylint would tell you this:
W0102: Dangerous default value [] as argument (dangerous-default-value)
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