Python, calling a function with default value for arguments

Question

In python, how can I call a function using the default value of an argument? I know it can be done just not passing the parameter when you call the function, but I would prefer to pass the variable through the function though. Having these two functions:

def A(argA1=defValA1, argA2=defValA2, argA3=defValA3):    
    argB1val=K*argA1
    argB2val=K*argA2
    argB3val=K*argA3
    B(argB1=argB1val, argB2=argB2val,argB3=argB3val)

def B(argB1=defValB1, argB2=defValB2, argB3=defValB3):
    print("First argument: "+argB1)
    print("Second argument: "+argB2)
    print("Third argument: "+argB3)

If I call A() with all the arguments, there’s no problem:

Main:

A(argA1val, argA2val, argA3val)

But what if I call A() with some missing arguments?:

Main:

A(argA1val)

In this case, inside the A() function

B() will be called taking the default values for the last two:

B(argB1= K*argA1, argB2= K* defValA2, argB3= K* defValA3)

However, I’d like to call B() using the default values of their last two arguments, something like :

B(argB1= K*argA1, argB2=defValB2, argB3=defValB3)

So, I would like A() to be something like:

def A(argA1=defValA1, argA2=defValA2, argA3=defValA3):

if (argA1==defValA1):
    argB1val=B.getDefaultValue("argB1")
else:
    argB1val=K*argA1

if (argA2==defValA2):
    argB2val=B.getDefaultValue("argB2")
else:
    argB2val=K*argA2

if (argA3==defValA3):
    argB3val=B.getDefaultValue("argB3")
else:
    argB3val=K*argA3

B(argB1=argB1val,argB2=argB2val,argB3=argB3val)

Otherwise, I’d need to do something like:

def A(argA1=defValA1, argA2=defValA2, argA3=defValA3):

if (argA1==defValA1 && argA2!=defValA2 && argA3!=defValA3):

    argB2val=K*argA2
    argB3val=K*argA3
    B(argB1= defValB1, argB2=argB2val,argB3=argB3val)

if (argA1!=defValA1 && argA2==defValA2 && argA3!=defValA3):

    argB1val=K*argA1
    argB3val=K*argA3
    B(argB1=argB1val, argB2= defValB2,argB3=argB3val)

if (argA1!=defValA1 && argA2!=defValA2 && argA3==defValA3):

    argB1val=K*argA1
    argB2val=K*argA2
    B(argB1= argB1val, argB2=argB2val,argB3= defValB1)

if (argA1==defValA1 && argA2==defValA2 && argA3!=defValA3):

    argB3val=K*argA3
    B(argB1= defValA1, argB2= defValA2,argB3=argB3val)

    ….
    ….
    ….

    Etc…

Having even a lot more combinations if we have more parameters.

Anyone knows if there’s a way to get the default value of a parameter to be passed in a function?

Thanks!

Your default values (defValA1, defValA2, defValA3) for the params of A, aren't appropriate enough to serve as default values, when you get to the part where you compute the args for invoking B from within A. A true default would be default in every sense. As suggested by @MatiasCicero, you might as well use some other values (such as None) as the defaults, in the place of defValA1, defValA2, and defValA3. — fountainhead

Matias Cicero Matias Cicero · Accepted Answer · 2019-02-20T23:54:43

Just let them be None and set them at execution time.

def A(argA1=None, argA2=None, argA3=None):    
    argB1val= (K * argA1) if argA1 is not None else defValA1
    argB2val= (K * argA2) if argA2 is not None else defValA2
    argB3val= (K * argA3) if argA3 is not None else defValA3
    B(argB1=argB1val, argB2=argB2val, argB3=argB3val)

def B(argB1=defValB1, argB2=defValB2, argB3=defValB3):
    print("First argument: " + argB1)
    print("Second argument: " + argB2)
    print("Third argument: " + argB3)

Python, calling a function with default value for arguments

1 Answers