Python: Hello, world! From zero to hero.

Coding a simple ‘hello world’ program

This blogpost contains some simple examples of hello-world programs in different languages:

• python
• TBD

Python

You can easily print a hello-world in Python by directly adding the following on a main.py file.

print("Hello, World!")

Execution:

$ python main.py

Preventing side-effects

You may have notice that the usage of the main.py python script as a module also triggers the hello-world statement.

import main  # This unintentonally prints the 'Hello,  World!' string

# ...

Adding the following validation prevents printing the hello-world side-effect on imports:

if __name__ == "__main__":
    print("Hello, World!")

Notice that the code still prints the ‘Hello, World!’ when running:

python main.py

Reusability

By now, we know how to execute the hello-world program and avoid any side-effects when using it as a module. Let’s refactor the code so that we can re-utilize the hello-world implementation on different appliccations:

def hello_world():
    print("Hello, World!")


if __name__ == "__main__":
    hello_world()
    

By implementing the hello_world function, we can now use this implementation in different scripts by importing the definition:

from main import hello_world  # Import without side-effects

# ...

Parametrization

We can parametrize the hello_world function so that we can select an arbitrary name.

def hello_world(name: str):
    print(f"Hello, {name}!")


if __name__ == "__main__":
    hello_world(name="World")

Default parameters

We can now define a default parameter to make the argument optional.

from typing import Optional


def hello_world(name: Optional[str] = None):
    name = name or "World"
    print(f"Hello, {name}!")
    
if __name__ == "__main__":
    hello_world()

Default Configurable parameters

import os
from typing import Optional


DEFAULT_WORLD = os.environ.get(
    "DEFAULT_WORLD",
    default="World"
)


def hello_world(name: Optional[str] = None):
    name = name or DEFAULT_WORLD
    print(f"Hello, {name}!")
    
if __name__ == "__main__":
    hello_world()

This allows us to change the default argument value:

$ python main.py

$ DEFAULT_WORLS=example python main.py

Commandline arguments

import argparse
import os
from typing import Optional


DEFAULT_WORLD = os.environ.get(
    "DEFAULT_WORLD",
    default="World"
)


def hello_world(name: Optional[str] = None):
    name = name or DEFAULT_WORLD
    print(f"Hello, {name}!")
    
    
if __name__ == "__main__":
    # Instance argument parser, configure and retrieve arguments
    parser = argparse.ArgumentParser(description="Hello World Program")
    parser.add_argument("--world", default=DEFAULT_WORLD)
    args = parser.parse_args()
    # Execute hello-world
    hello_world(name=args.world)