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Mutable objects and references

Lists and dictionaries are mutable: when passed into a function, the function works on the same, shared list — changes are visible outside too. Numbers, strings and tuples are immutable and can never be changed afterwards, only replaced.

f(x) \Rightarrow x \text{ changes permanently, if } x \text{ is mutable}

e.g. list.append

Symbols

x

the object passed to the function

Example

def f(x): x.append(4)

a = [1, 2, 3]; f(a)

len(a) is now 4, because a and x point to the same list.

To keep the original unchanged, pass a copy: f(a.copy()).

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Part of Programming with Python: Functions and data structures.