How does python handle exceptional situations?

Question

I learnt about using the except function to prevent your code from crashing but I want to understand it better on how python executes it.

For example, I have code that adds a pair of numbers and if the variable nums has more than two numbers it should give AssertionError.

1    def sum_pair(pair):
2        assert len(pair) == 2
3        return pair[0] + pair[1]

4    try:
5        total = sum_pair(nums)
6        print 'The total is', total
7    except AssertionError:
8        print 'Abort'

So from my view python executes Lines 1, 2, 4, 5, 7 and 8 only. Is this correct way to understand it?

Thank you

The statements inside a function aren't executed until you call the function. So line 2 won't be executed until after line 5. — Barmar
– Barmar, Commented May 16, 2016 at 21:57

Barmar · Accepted Answer · 2016-05-16 22:04:21Z

2

When an exception is signalled, the call stack is searched for a try block that has an except clause that matches the type of the exception. The stack is unwound to that point, the code in the except clause is executed, and then execution continues after the try block.

So when line 2 detects that the assertion has failed, it immediately exits the sum_pair() function and executes the code starting from line 8.

answered May 16, 2016 at 22:04

Barmar

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Comments

Srini · Accepted Answer · 2016-05-16 21:46:16Z

1

Technically, it executes Lines 4,5,1,2,7,8 in that order.

answered May 16, 2016 at 21:46

Srini

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3 Comments

TigerhawkT3 Over a year ago

I'm pretty sure the OP realizes that and was just sorting the executed line numbers. Additionally, this answer would be more suitable as a comment.

Srini Over a year ago

@TigerhawkT3, Fair enough. When I read the question, I interpreted it as OP wanting to know if the execution order was the correct way to understand it, but your point is well taken.

Serge Ballesta Over a year ago

IMHO, the def line is executed first, so that the identificator sum_pair is knows to the interpretor. So the order of execution is 1,4,5,2,7,8

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