Python Forum

Hello, I've stumbled across a problem when creating a program.
The start to my program looks along the lines of:

def parseVote(s):
    if s == []:
         print('0')  

If the user doesn't input a value for parseVote() I want it to print '0' but it is missing an argument so wont do that, help would be greatly appreciated!

You need to make the argument optional, i.e. provide default value:

def parseVote(s=None):
    if not s: # no argument or 0 or empty list/tuple as argument
        print(0)
    else:
        print(s)

In this example the default value is None, but it can be some number, e.g. 0, which will simplify the code.

def parseVote(s=0):
    print(s) # print 0 if no argument but prints empty list/tuple if supplied as argument

Note that you should not supply mutable default arguments, e.g.

def parseVote(s=[]):

THAT'S NOT THE RIGHT WAY

You don't know how long I have messed around with this, thank you God!

(Apr-27-2017, 07:26 AM)buran Wrote: [ -> ]Note that you should not supply mutable default arguments, e.g.

def parseVote(s=[]):

THAT'S NOT THE RIGHT WAY

Actually, in some very specific cases this is the right way - if you build a caching mechanism. Though in those cases I would use underscored name.

*args ?

What do you know - I got something right

Yes, but that's very specific case, when this is actually desired result (i.e. you know what you are doing) and this is also mentioned in the link I provided -

Quote:When the Gotcha Isn’t a Gotcha

Sometimes you can specifically “exploit” (read: use as intended) this behavior to maintain state between calls of a function. This is often done when writing a caching function.

(Apr-27-2017, 11:56 AM)buran Wrote: [ -> ]Yes, but that's very specific case, when this is actually desired result (i.e. you know what you are doing) and this is also mentioned in the link I provided -

I followed the link after I replied.....

(Apr-27-2017, 11:26 AM)volcano63 Wrote: [ -> ]Actually, in some very specific cases this is the right way - if you build a caching mechanism. Though in those cases I would use underscored name.

I've only ever seen it as a gotcha, including for experienced Python programmers, and have never actually seen this used for caching or anything like that (and I'd be reluctant to accept it as a good use case).

(Apr-27-2017, 05:02 PM)micseydel Wrote: [ -> ]I've only ever seen it as a gotcha, including for experienced Python programmers, and have never actually seen this used for caching or anything like that (and I'd be reluctant to accept it as a good use case).

Well, I have some experience, and though I've never seen it used by anyone else, I used it a couple of time. Does improving performance by 100-s of percents in resource-hogging time-critical apps count as a good cause(yes, occasionally improving performance counts, even in Python   ).

Of course, you have to keep in mind

• Cache maintenance - you may easily end up with memory leak
  
• You cannot re-assign cache - you may clean it, but you must preserve the reference
  

I am not sure if 4 years of Python (first time I used this technique couple of years ago) would define me as experienced Python programmer   , but I have some experience with programming...