def myF(a, b):
return a*b-2*b
Let's say that I want a default value for b to be a-1:
def myF(a, b=a-1):
return a*b-2*b
gets the error message:
NameError: name 'a' is not defined
I can use the code below:
def myF(a, b):
return a*b-2*b
def myDefaultF(a):
return myF(a, a-1)
to have myF with default value, but I don't like it.
How can I avoid myDefaultF and have myF with default value a-1 for b without errors?
You can do the following:
def myF(a, b=None):
if b is None:
b = a - 1
return a * b - 2 * b
If you need to have the value of b be a function of a, but you might need that function to change, you can set the default value of b to be a lambda function and then check if b is callable in the function block.
def myF(a, b=lambda a: a-1):
if callable(b):
b = b(a)
return a * b - 2 * b
This allows you to set a different function for b on the fly as well.
# pass b as an integer
myF(1, 1)
# returns: -1
# use default function for b
myF(4)
# returns: 6
# set b to be 2*a + 1
myF(3, lambda a: 2*a+1)
# returns: 7
You can use a try-except clause:
def myF(a, b=None):
try:
return a*b-2*b
except:
return a*(a-1)-2*(a-1)
except clause will catch and silently suppress other errors, such as passing a non-numeric value for b. From The Zen of Python: "Explicit is better than implicit".