13.2. Defining default values

13.2. Defining default values
	Chapter 13. Functions II

Default values of parameters can be defined in the function definition.

Example 13.1. Default values of parameters

Say that you have a function that checks whether a sequence is correct with respect to a given alphabet.

def checkseq(seq, alphabet):
    for c in seq:
       if c not in alphabet:
          return False
    return True

To use the DNA alphabet as default value for the alphabet parameters, the checkseq function can be redefined as follow:

def checkseq(seq, alphabet="actg"):
    for c in seq:
       if c not in alphabet:
          return False
    return True

So, you can now call it this way:

>>> result = checkseq("atgcgtgatgdtgragt")
False
>>> myseq = "agcucgaua"
>>> result = checkseq(myseq, "acgu")
True

Default values are referenced when the function is defined.

alphabet = "actg"
def checkseq(seq, alphabet=alphabet):
    for c in seq:
       if c not in alphabet:
          return False
    return True

If you change the global alphabet variable later, this will not be taken into account, except if the default value is a mutable type of course (e.g: list or dictionary).

alphabet = "actg"
def checkseq(seq, alphabet=alphabet):
    for c in seq:
       if c not in alphabet:
          return False
    return True

alphabet = "acug"
>>> result = checkseq("atgcgtgatgdtgragt")
True