[Python] Pitfalls: About Default Parameter Values in Functions

Today an interesting bug (pitfall) is found when I was trying debug someone's code. There is a function which tries to check if an object exists or not. It has several parameters and some of them have default value. When using this function, the programmer intended to give some values to some certain parameters but keep the rest for default values. However I astonishedly found that the passed parameters are not those as we intended to pass. For example, we have a function defined like this:

def printDefaultParas(para_1, para_2 = False, para_3 = False):
    print("para_1=%s, para_2=%s, para_3=%s"%(para_1, para_2, para_3))

Then, we have some other functions called it, for example one of them could be:

def pitfallHappens():
    para_1 = "This is Para 1"
    para_3 = True
    printDefaultParas(para_1, para_3)

What we expected (or we intended) to have is, the function prints out:

para_1=This is Para 1, para_2=False, para_3=True

But what we actually got will be:

para_1=This is Para 1, para_2=True, para_3=False

Why?

In fact, it is because, in the function pitfallHappens(), when we call function printDefaultParas(), these para_1, and para_3 we put in the parameter list, do not stand for the parameters at all. They are just variables happened to be the same name as the parameter names! As a result, Python will put this values accordingly in the sequence of the parameter list, and leave the rest as their default values (if possible). In this case, it gives the variable para_1's value ("This is Para 1") to parameter para_1, and variable para_3's value (True) to parameter para_2, and leave parameter para_3 as its default value (False).

Now we are clear, and function pitfallHappens() could be corrected as:

def pitfallCorrected():
    para_1 = "This is Para 1"
    para_3 = True
    printDefaultParas(para_1 = para_1, para_3 = para_3)

Then pitfallCorrected() will get expected result.