字符串未从函数返回，但返回了列表

Question

I use a function to aggregate strings;
this function performs this concatenation via 2 different methods:
1) via string concatenation : CUM_STR = CUM_STR + str(IDX)
2) via List append : CUM_LST.append(IDX)

When returning from this function,
method 1 always gives an empty string for CUM1_TXT
method 2 correcly aggregates strings in the CUM_LST List

Here is a sample of this case :

CUM1_TXT = ''
CUM1_LIST = []

def modif(IDX,CUM_STR,CUM_LST):
    CUM_STR = CUM_STR + str(IDX)
    CUM_LST.append(IDX)
    print CUM_STR
    print CUM_LST
    return CUM_STR,CUM_LST

for INDEX in range(10):
    modif(INDEX,CUM1_TXT,CUM1_LIST)
    print CUM1_TXT,CUM1_LIST

By the way, the fact to code the CUM_STR and CUM_LST on the return statement does not change anything at the result

Any help appreciated

Answer 1

let's take your method:

def modif(IDX,CUM_STR,CUM_LST):
    CUM_STR = CUM_STR + str(IDX)
    CUM_LST.append(IDX)
    print CUM_STR
    print CUM_LST
    return CUM_STR,CUM_LST

CUM_STR is a str object. It is immutable , so modif cannot change the caller value.

CUM_LST is a list object. It is mutable . Appending within the method affects the caller value (altough not very nice to modify parameters, caller may not expect it)

and to top it all, you're returning the values, but you don't assign them in the caller: return values are lost on function return. Performing a return on parameters does not change the parameters.

If you allow me, I would rewrite it like this:

def modif(IDX,CUM_STR,CUM_LST):
    return CUM_STR + str(IDX),CUM_LST+[IDX]

and you'd have to call it like this:

CUM1_TXT,CUM1_LIST = modif(INDEX,CUM1_TXT,CUM1_LIST)

Answer 2

You are discarding the return value of modif function, replace the call to modif with this line:

CUM1_TXT, _ = modif(INDEX,CUM1_TXT,CUM1_LIST)

Note that "_" is Python common practice to call variable which will not be used

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