Modularity

Splitting a program: one function per task, factoring out repeated code, grouping functions into modules, importing, and separating the main program from what can be reused.

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Modularity: what is it?
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A way of building a program in separate pieces, each with a precise role, which can be written, tested and changed independently of each other.
A defect is looked for in one piece only, an improvement touches only that piece, and a piece already written is reused in another program.
What does python3 main.py print?

```python
# outils.py
print("loading")

def double(n):
return 2 * n

if __name__ == "__main__":
print("test", double(4))

# main.py
import outils
print(outils.double(5))
`
- loading, then 10
- 10 alone
- loading, test 8, then 10
- test 8, then 10
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loading, then 10
Importing runs the file: the file-level print is displayed. But __name__ is then "outils" and not "__main__", so the test block is skipped.
Repeated code is replaced by a …: if a defect is fixed, it is fixed … only.
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Repeated code is replaced by a function: if a defect is fixed, it is fixed once only.
The three copies of a 10% discount become a single appliquer_remise(prix), and changing the rate is done in one place.
Module: what is it?
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A file that groups together related functions and values, and which another file can use by importing it.
In Python, any .py file is a module: outils.py is imported under the name outils. Grouping aire and perimetre in geometrie.py says where to look for them.
Which of these functions best respects single responsibility?
- calculer_moyenne(notes)
- lire_et_afficher_moyenne()
- traiter_notes_et_ecrire_fichier(notes)
- programme_complet()
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calculer_moyenne(notes)
calculer_moyenne has one task and can be tested without keyboard or file. The other three mix reading, computing, displaying or writing: changing one of these tasks forces you to reopen the others.

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