Python - Default Arguments

Python allows to define a function with default value assigned to one or more formal arguments. Python uses the default value for such an argument if no value is passed to it. If any value is passed, the default value is overridden with the actual value passed.

Default arguments in Python are the function arguments that will be used if no arguments are passed to the function call.

Example of Default Arguments

The following example shows use of Python default arguments. Here, the second call to the function will not pass value to “city” argument, hence its default value “Hyderabad” will be used.

# Function definition 
def showinfo( name, city = "Hyderabad" ):
   "This prints a passed info into this function"
   print ("Name:", name)
   print ("City:", city)
   return

# Now call showinfo function
showinfo(name = "Ansh", city = "Delhi")
showinfo(name = "Shrey")

It will produce the following output −

Name: Ansh
City: Delhi
Name: Shrey
City: Hyderabad

Example: Calling Function Without Keyword Arguments

Let us look at another example that assigns default value to a function argument. The function percent() has a default argument named “maxmarks” which is set to 200. Hence, we can omit the value of third argument while calling the function.

# function definition 
def percent(phy, maths, maxmarks=200):
   val = (phy + maths) * 100/maxmarks
   return val

phy = 60
maths = 70
# function calling with default argument
result = percent(phy, maths)
print ("percentage:", result)

phy = 40
maths = 46
result = percent(phy, maths, 100)
print ("percentage:", result)

On executing, this code will produce the following output −

percentage: 65.0
percentage: 86.0

Mutable Objects as Default Argument

Python evaluates default arguments once when the function is defined, not each time the function is called. Therefore, If you use a mutable default argument and modify it within the given function, the same values are referenced in the subsequent function calls.

Those Python objects that can be changed after creation are called as mutable objects.

Example

The code below explains how to use mutable objects as default argument in Python.

def fcn(nums, numericlist = []):
   numericlist.append(nums + 1)
   print(numericlist) 
    
# function calls
fcn(66)
fcn(68)
fcn(70)

On executing the above code, it will produce the following output −

[67]
[67, 69]
[67, 69, 71]