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Definition and Invocation of Functions

Functions are used to encapsulate and isolate code logic. In Python and JavaScript, functions are considered first-class citizens.

Task​

Write a function that greets a person by formatting their personal information (name, age, gender) and printing it.

JavaScript implementation​

function greet(name, age, gender='Secret') {
  console.log(`Hello, I am ${name}, ${age} years old, ${gender}`)
}

greet('Bob', 18);
greet('Tom', 18, 'male');
greet(...['Amy', 18, 'female']);

Python implementation​

def greet(name, age, gender='Secret'):
    print(f"Hello, I am {name}, {age} years old, {gender}")


greet('Bob', 18)  # Positional arguments
greet(age=18, name='Bob')  # Keyword arguments
greet('Tom', 18, 'male')
greet(*['Amy', 18, 'female'])

Code Highlight​

  • In Python, functions are defined using the def keyword, while in JavaScript, the function keyword is used.
  • In Python, the function body is indicated by a colon : and indentation, while in JavaScript, it is enclosed in curly braces.
  • In Python, every parameter of a function, except for those with default values (like the gender parameter above), is required. If a required parameter is not provided, a syntax error will occur. In JavaScript, parameters that are not passed are assigned the value undefined by default.
  • In Python, parameters passed in the order they are defined are called positional arguments, and they must be passed in the exact order as defined in the function signature. Additionally, keyword arguments can be passed using the key=value syntax. The order of keyword arguments does not need to match the order of the parameters. Positional and keyword arguments can be used together, but all necessary arguments must be provided when calling the function.
  • In Python, positional arguments can be passed using the syntax my_fn(*[p1, p2, ...]), similar to myFn(...[p1, p2, ...]) in JavaScript.

Difference Quick View​

FeatureJavaScriptPython
Function Definitionfunction myFn(params) { ... }def my_fn(params):
Function InvocationmyFn(params)my_fn(params)
How to get function arguments?

Use *args to get all positional arguments passed to the function. The arguments will be stored in a tuple. Use **kwargs to get all keyword arguments passed to the function. The arguments will be stored in a dictionary.

def some_fn(position_arg, *args, **kwargs):
    print(position_arg)
    print(args)
    print(kwargs)

some_fn('position_arg_value','add_to_args1','add_to_args2',a=1,b=2,c=3)
# Output
# position_arg_value
# ('add_to_args1', 'add_to_args2')
# {'a': 1, 'b': 2, 'c': 3}

Similarly, Python can expand a dictionary using **.

def some_fn(a, b, c):
    print(a, b, c) # 1 2 3

my_dict = {"c": 3, "b": 2, "a": 1} # Python has no requirement for the order of keys in a dictionary
some_fn(**my_dict)

While *args and **kwargs are convenient, if a function only has these parameters, IDE code suggestions will be lost, and the logic of the function may become harder to understand. Therefore, use them with caution.

Resources​