[FIXED] How do I use all() built-in function?

Issue

I am attempting to use all() but it is not working for me:

>>> names = ["Rhonda", "Ryan", "Red Rackham", "Paul"]
>>> all([name for name in names if name[0] == "R"])
True
>>> 

I am trying to check if all the names begin with "R", and even though I added "Paul" to names, all() still returns True. How do I fix this so that all() returns False until "Paul" is removed?

Solution

You misunderstand how all works. From the docs:

all(iterable)

Return True if all elements of the iterable are true (or if the iterable is empty).

In your code, you are first collecting all names that start with R into a list and then passing this list to all. Doing this will always return True because non-empty strings evaluate to True.

---

Instead, you should write:

all(name[0] == "R" for name in names)

This will pass an iterable of booleans to all. If all of them are True, the function will return True; otherwise, it will return False.

As an added bonus, the result will now be computed lazily because we used a generator expression instead of a list comprehension. With the list comprehension, the code needed to test all strings before determining a result. The new code however will only check as many as necessary.

Answered By - user2555451