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

It's perfectly safe and I do it all the time.

Answer 3

I would go with the try/except, but it depends on what you know about the variable.

If you are expecting that the variable will exist most of the time, then a try/except is less operations. If you are expecting the variable to be None most of the time, then an IF statement will be less operations.

Answer 4

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.

Answer 5

Guys its not that simple. As a C# dud I am very used to doing something like

if(x != null && ! string.isnullorempty(x.Name))

{ //do something }

The above works great and is evaluated as expected. However in VB.Net the following would produce a result you where NOT expecting

If Not x Is Nothing And Not String.IsNullOrEmpty(x.Name) Then

'do something

End If

The above will generate an exception. The correct syntax should be

If Not x Is Nothing AndAlso Not String.IsNullOrEmpty(x.Name) Then

'do something

End If

Note the very subtle difference. This had me confused for about 10 minutes (way too long) and is why C# (and other) dudes needs to be very carefull when coding in other languages.

Answer 6

I may be being a little pedantic here, but I would say the best answer is

if my_var is not None and 'something' in my_var:
    #do something

The difference being the explicit check for None, rather than the implicit conversion of my_var to True or False.

While I'm sure in your case the distinction isn't important, in the more general case it would be quite possible for the variable to not be None but still evaluate to False, for example an integer value of 0 or an empty list.

So contrary to most of the other posters' assertions that it's safe, I'd say that it's safe as long as you're explicit. If you're not convinced then consider this very contrived class:

class Contrived(object):
    def __contains__(self, s):
        return True
    def __nonzero__(self):
        return False

my_var = Contrived()
if 'something' in my_var:
    print "Yes the condition is true"
if my_var and 'something' in my_var:
    print "But this statement won't get reached."
if my_var is not None and 'something' in my_var:
    print "Whereas this one will."

Yes I know that's not a realistic example, but variations do happen in real code, especially when None is used to indicate a default function argument.