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# Python Function Tutorial – Part VI

###### Posted April 22, 2020 at 9:28 am
QuantInsti

Get up-to-speed with the prior articles in this series:  Part IPart II , Part III , Part IV and Part V.

Python functions with default arguments

Let us say, we are writing a function that takes multiple parameters. Often, there are common values for some of these parameters. In such cases, we would like to be able to call the function without explicitly specifying every parameter. In other words, we would like some parameters to have default values that will be used when they are not specified in the function call.

To define a Python function with a default argument value, we need to assign a value to the parameter of interest while defining a function.

def power(number, pow=2):
“””Returns the value of number to the power of pow.”””
return number**pow

Notice that the above python function computes the first argument to the power of the second argument. The default value of the latter is 2. So now when we call the python function `power` only with a single argument, it will be assigned to the `number` parameter and the return value will be obtained by squaring `number`.

# Calling the power function only with required argument
print(power(2))
# Output
4

In other words, the argument value to the second parameter `pow` became optional. If we want to calculate the number for a different power, we can obviously provide a value for it and the python function will return the corresponding value.

# Calling the power function with both arguments
print(power(2, 5)
# Output
32

We can have any number of default value parameters in a python function. Note however that they must follow non-default value parameters in the definition. Otherwise, Python will throw an error as shown below:

# Calling the power function that will throw an error
def power(pow=2, number):
“””Returns the raised number to the power of pow.”””
return number**pow
File ““, line 1
def power(pow=2, number):
^
SyntaxError: non-default argument follows default argument

In the next installment, the author will discuss Python functions with variable length arguments.

Visit https://www.quantinsti.com/ for ready-to-use Python functions as applied in trading and data analysis.

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