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Q&A How and where does Python code start running? How can I control that?

Files with .py extension are scripts. You run them with python myscript.py. Python is an imperative language, so executing a file will run each line one by one, starting from the top, and exit whe...

posted 8mo ago by matthewsnyder‭ · edited 6mo ago by Michael‭

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#2: Post edited by

Michael‭ · 2024-06-17T20:00:40Z (6 months ago)
Fix typo. Markdown headings.

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Files with `.py` extension are *scripts*. You run them with `python myscript.py`.
Python is an imperative language, so executing a file will run each line one by one, starting from the top, and exit when the end of the file is reached.
In your example, the script is saying:
~~1. Create a function called `my_function`~~
~~2. This function, when called, will print some text~~
Notice we're telling Python to *create* the function, but we never tell it to call that function. Your example will appear to do nothing when executed (well, it may briefly waste some memory to store this function which never not gets used).
~~You have two options here, either get rid of the function definition, or add call.~~
~~Option 1:~~
~~```~~
# an actual command, not the content of a function definition
print("Test")
```
~~Option 2:~~
~~```~~
# define a function
def my_function():
print("Test")
# actually execute the function
my_function()
```

Files with `.py` extension are *scripts*. You run them with `python myscript.py`.
Python is an imperative language, so executing a file will run each line one by one, starting from the top, and exit when the end of the file is reached.
In your example, the script is saying:
1. Create a function called `my_function`.
2. This function, when called, will print some text.
Notice we're telling Python to *create* the function, but we never tell it to *call* that function. Your example will appear to do nothing when executed. (Well, it may briefly waste some memory to store this function, which never gets used.)
You have two options here: either get rid of the function definition, or add a call to execute it.
### Option 1:
```py
# an actual command, not the content of a function definition
print("Test")
```
### Option 2:
```py
# define a function
def my_function():
print("Test")
# actually execute the function
my_function()
```

#1: Initial revision by

matthewsnyder‭ · 2024-04-25T17:17:46Z (8 months ago)

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Files with `.py` extension are *scripts*. You run them with `python myscript.py`.

Python is an imperative language, so executing a file will run each line one by one, starting from the top, and exit when the end of the file is reached.

In your example, the script is saying:

1. Create a function called `my_function`
2. This function, when called, will print some text

Notice we're telling Python to *create* the function, but we never tell it to call that function. Your example will appear to do nothing when executed (well, it may briefly waste some memory to store this function which never not gets used).

You have two options here, either get rid of the function definition, or add call.

Option 1:
```
# an actual command, not the content of a function definition
print("Test")
```

Option 2:
```
# define a function
def my_function():
    print("Test")

# actually execute the function
my_function()
```

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