How can I check that a list has one and only one truthy value?

Question

In python, I have a list that should have one and only one truthy value (that is, bool(value) is True). Is there a clever way to check for this? Right now, I am just iterating across the list and manually checking:

def only1(l)     true_found = False     for v in l:         if v and not true_found:             true_found=True         elif v and true_found:              return False #"Too Many Trues"     return true_found 

This seems inelegant and not very pythonic. Is there a cleverer way to do this?

Answer 1

One that doesn't require imports:

def single_true(iterable):     i = iter(iterable)     return any(i) and not any(i) 

Alternatively, perhaps a more readable version:

def single_true(iterable):     iterator = iter(iterable)      # consume from "i" until first true or it's exhausted     has_true = any(iterator)       # carry on consuming until another true value / exhausted     has_another_true = any(iterator)       # True if exactly one true found     return has_true and not has_another_true 

This:

• Looks to make sure i has any true value
• Keeps looking from that point in the iterable to make sure there is no other true value

Answer 2

It depends if you are just looking for the value True or are also looking for other values that would evaluate to True logically (like 11 or "hello"). If the former:

def only1(l):     return l.count(True) == 1 

If the latter:

def only1(l):     return sum(bool(e) for e in l) == 1 

since this would do both the counting and the conversion in a single iteration without having to build a new list.