12. Functions and default arguments

Functions are declared with `def`. Parameters can have default values, and calls can use positional or keyword arguments.

python
def mint(amount, decimals=18):
    return amount * 10 ** decimals

mint(2)             # uses default decimals
mint(2, decimals=6) # keyword override

A function without an explicit `return` returns `None`. Mutable default arguments (like `[]` or `{}`) are a classic pitfall because they are created once and shared across calls — prefer `None` and create the mutable object inside the function.

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