Update: You can find anewer version of this article on Python Morsels.

In this article, we’re going to look at a common programming pattern and discuss how we can refactor our code when we notice this pattern. 🏗

We’ll be discussing how to make code with this shape a little more descriptive:

12345

all_good=Trueforiteminiterable:ifnotcondition(item):all_good=Falsebreak

An Example: Primality

Here’s a function that checks whether a given number is prime by trying to divide it by all numbers below it:

12345

defis_prime(candidate):forninrange(2,candidate):ifcandidate%n==0:returnFalsereturnTrue

Note: asquare root makes this faster and our code breaks below2 but we’ll ignore those issues here

This function:

1. loops from 2 to the given number
2. returnsFalse as soon as a divisor is found
3. returnsTrue if no divisor was found

This primality check is asking “do any numbers evenly divide the candidate number”.

Note that this functionreturns as soon as it finds a divisor, so itonly iterates all the way through the number range when the candidate number is prime.

Let’s take a look at how we can rewrite this function usingall.

What’sall?

Python has a built-in functionall that returnsTrue if all items aretruthy

12345678

>>>all(['hello, 'there'])True>>>all(['hello, 'there', ''])False>>>all([1,2,3])True>>>all([0,1,2,3])False

You can think oftruthy as meaning non-empty or non-zero. For our purposes, we’ll treat it as pretty much the same asTrue.

Theall built-in function is equivalent to this:

12345

defall(iterable):forelementiniterable:ifnotelement:returnFalsereturnTrue

Notice the similarity betweenall and ouris_prime function? Ouris_prime function is similar, but they’re not quite the same structure.

Theall function checks for the truthiness ofelement, but we need something a little more than that: we need to check a condition on each element (whether it’s a divsior).

Usingall

Our originalis_prime function looks like this:

12345

defis_prime(candidate):forninrange(2,candidate):ifcandidate%n==0:returnFalsereturnTrue

If we want to useall in this function, we need an iterable (like a list) to pass toall.

If we wanted to be really silly, we could make such a list of boolean values like this:

12345678

defis_prime(candidate):divisibility=[]forninrange(2,candidate):ifcandidate%n==0:divisibility.append(False)else:divisibility.append(True)returnall(divisibility)

We could simplify this function like this:

12345

defis_prime(candidate):divisibility=[]forninrange(2,candidate):divisibility.append(candidate%n!=0)returnall(divisibility)

I know this is probably doesn’t seem like progress, but bear with me for a few more steps…

List comprehensions

If you’re familiar withlist comprehensions, this code structure might look a little familiar. We’re creating one iterable from another which is exactly what list comprehensions are good for.

Let’s copy-paste our way into a list comprehension (see my article onhow to write list comprehensions):

123456

defis_prime(candidate):divisibility=[candidate%n!=0forninrange(2,candidate)]returnall(divisibility)

That’s quite a bit shorter, but there’s a problem: we’rebuilding up an entire list just to loop over it once!

This is less efficient than our original approach, which only looped all the way whencandidate was prime.

Let’s fix this inefficiency by turning our list comprehension into a generator expression.

Generator expressions

Agenerator expression is like a list comprehension, but instead of making a list it makes agenerator object.

A generator is aniterator: generators don’t compute the items they contain until you loop over them. We’ll see what that means in a moment.

We can turn our list comprehension into a generator expression by changing the brackets to parentheses:

123456

defis_prime(candidate):divisibility=(candidate%n!=0forninrange(2,candidate))returnall(divisibility)

Now our code doesn’t create a list to loop over. Instead it provides us with a generator that allows us to compute the divisibility of each number one-by-one.

We can make this code even more readable by putting that generator expression inside the function call (notice that we can drop the second set of parentheses):

12345

defis_prime(candidate):returnall(candidate%n!=0forninrange(2,candidate))

Note that because our generator is lazy, we stop computing divisibilities as soon as ourall function finds a divisible number. So we end up calculatingcandidate % n != 0 only as many times as we did in our original function.

Recap

So we started with afor loop, anif statement, areturn statement for stopping once we find a divisor, and areturn statement for the case where our number had no divisors (when it’s prime).

12345

defis_prime(candidate):forninrange(2,candidate):ifcandidate%n==0:returnFalsereturnTrue

We turned all that into a generator expression passed to theall function.

12345

defis_prime(candidate):returnall(candidate%n!=0forninrange(2,candidate))

I prefer this second approach (a generator expression withall) because I find itmore descriptive.

We’re checking to see whether “all numbers in a range are not divisors of our candidate number”. That sounds quite a bit more like English to me than “loop over all numbers in a range and return False if a divisor is found otherwise return True”.

If you don’t find the behavior ofall intuitive, you might find it easier to understand (and more English-like) when used withif:

1234

ifall(condition(item)foriteminiterable):message="All good"else:message="Bad value found"

You can always reformat your code to use anif statement if you find it more readable.

any orall

We’ve been working with theall function, but I haven’t mentioned it’s counterpart: theany function. Let’s take a look at howall andany compare.

These two expressions:

12345678

all_good=all(condition(x)forxinthings)some_bad=notall(condition(x)forxinthings)

Are equivalent to these two expressions (because ofDeMorgan’s Laws):

12345678

all_good=notany(notcondition(x)forxinthings)some_bad=any(notcondition(x)forxinthings)

So this code:

12345

defis_prime(candidate):returnall(candidate%n!=0forninrange(2,candidate))

Is feature-identical to this code:

12345

defis_prime(candidate):returnnotany(candidate%n==0forninrange(2,candidate))

Both of them stop as soon as they find a divisor.

I find the use ofall more readable here, but I wanted to mention thatany would work just as well.

Cheat sheet for refactoring withany andall

All that explanation above was valuable, but how can we use this new knowledge to refactor our own code? Here’s a cheat sheet for you.

Anytime you see code like this:

12345

all_good=Trueforiteminiterable:ifnotcondition(item):all_good=Falsebreak

You can replace that code with this:

1234

all_good=all(condition(item)foriteminiterable)

Anytime you see code like this:

12345

any_good=Falseforiteminiterable:ifcondition(item):any_good=Truebreak

You can replace it with this:

1234

any_good=any(condition(item)foriteminiterable)

Note thatbreak is used in the code above because we’re not returning from a function. Usingreturn (like we did inis_prime) is another way to stop our loop early.

Python’sany andall functions weremade for use with generator expressions (discussionhere andhere). You can useany andall without generator expressions, but I don’t find a need for that as often.

Quick note:any(item == 'something' for item in iterable) is the same as'something' in iterable. Don’t useall/any for checking containment, usein.

Code style is a process

As you discover new Python idioms and new language features are invented, your code style will evolve. Your preferred code style may never stop evolving. Code style is not concrete: it’s a process.

I hope I’ve inspired you to embrace the use ofany/all with generator expressions for improved readability and code clarity.

If you’d like toget practice with theany andall functions right now, sign up for Python Morsels using the form below to get an exercise that benefits from using one of these two functions.

I made Python Morsels tohelp experienced programmers level up their Python skills every week.For more details on it, [see the Python Morsels][python morsels] website.