python order of evaluation with ands in if statements

Question

It's common to put in a check for None or have similar error catch. Sometimes you also need another if statement that depends on x:

if x == None:
   if x[0]>0:
     ... # some code

Can you safely combine the if statements like this (for code brevity)?

if x != None and x[0]>0:
     ... # some code

Or does the interpreter not guarantee the order of evaluation and stopping after the first False?

Answer 1

Yes it is safe, because the operators and and or short-circuits.

Note:

• One recommendation would be to use is if you want to check if an object is None:

  if x is not None and x[0] > 0:
      # ...
  

Answer 2

Yes, because Python and is short-circuited (wikipedia) and evaluated from left to right.

That means if x == None, it won't evaluate the rest: Python already knows that the evaluation of the full and is going to be False, because: False and True and True and True is still False

Consider the following:

x = None    
if x is not None and a != "test":
    print "Point 1"
else:
    print "Point 2"

The variable a is not initialized anywhere, right? If Python hits that point, you'll get a Name Error exception, but what you get is:

Point 2

Meaning that Python doesn't even bother evaluating the a != "test" part.

Something similar happens with or. If any condition is True, Python stops evaluating:

The following code:

x = None
if x is None or a != "test":
    print "Point 1"
else:
    print "Point 2"

Doesn't throw an exception either, but outputs

Point 1

Bonus:

When checking if a variable is None, is much more recommended using is (or is not) rather than == or != (as explained in the SO question What is the difference between " is None " and " ==None ")