Is there a way to use a method/function as an expression for .loc() in pandas?

I've been going crazy trying to figure this out. I'm trying to avoid using df.iterrows() to iterate through the rows of a dataframe, as it's quite time consuming and .loc() is better from what I've seen.

I know this works:

df = df.loc[df.number == 3, :]

And that'll basically set df to be each row where the "number" column is equal to 3.

But, I get an error when I try something like this:

df = df.loc[someFunction(df.number), :]

What I want is to get every row where someFunction() returns True whenever the "number" value of said row is set as the parameter.

For some reason, it's passing the entire column (the dataframe's entire "number" column, in this example), instead of the value of a row as it iterates through the row, like the previous example.

Again, I know I can just use a for loop and .iterrows(), but I'm working with around 280,000 rows and it just takes longer than I'd like. Also have tried using a lambda function among other things.