Is it safe to rely on condition evaluation order in if statements?

Question

Is it bad practice to use the following format when my_var can be None?

if my_var and 'something' in my_var:     #do something 

The issue is that 'something' in my_var will throw a TypeError if my_var is None.

Or should I use:

if my_var:     if 'something' in my_var:         #do something 

or

try:     if 'something' in my_var:         #do something except TypeError:     pass 

To rephrase the question, which of the above is the best practice in Python (if any)?

Alternatives are welcome!

Answer 1

It's safe to depend on the order of conditionals (Python reference here), specifically because of the problem you point out - it's very useful to be able to short-circuit evaluation that could cause problems in a string of conditionals.

This sort of code pops up in most languages:

IF exists(variable) AND variable.doSomething()     THEN ... 

Answer 2

Yes it is safe, it's explicitly and very clearly defined in the language reference:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.