Assertions and preconditions

Stating what a function assumes and what it guarantees: precondition, postcondition, invariant, the assert statement and its failure, defensive programming, and the difference with checking an input.

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Precondition: what is it?
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A condition that must hold when a function is called for it to be sure to work properly.
It binds the caller: if racine(n) assumes n >= 0, the caller must not pass it -4.
What does this program print?

```python
def racine(n):
assert n >= 0, "n must be zero or more"
return n ** 0.5

print(racine(9))
print(racine(-4))
print("end")
`
- 3.0, then AssertionError: n must be zero or more, without end
- 3.0, then the computation of (-4) ** 0.5 runs, then end is displayed
- 3.0, n must be zero or more, then end
- 3.0, then a ValueError raised on the return line
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3.0, then AssertionError: n must be zero or more, without end
assert raises AssertionError with the message when the condition is false, and nothing runs after it: neither the return nor print("end"). The stop happens at the assertion's line, not in the computation.
A … must hold before the call, a … is guaranteed afterwards.
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A precondition must hold before the call, a postcondition is guaranteed afterwards.
Together they form the function's contract: if the caller meets the precondition, the function guarantees the postcondition.
Postcondition: what is it?
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A condition the function guarantees about its result when it finishes, provided its preconditions were met.
It binds the function: for trier(t), the result holds the same elements as t, in ascending order.
A program asks the user for an age and passes it to a function that assumes age >= 0. What should be done with the input?
- Check it with an if and ask again
- Write assert age >= 0 and let the program stop if the user makes a mistake
- Check nothing: the function's precondition is enough to protect the program
- Replace any negative value with 0 without warning
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Check it with an if and ask again
A faulty input is an expected case: it is handled with an if and a message. The assertion guards against programming errors, and Python can ignore it with the -O option.

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