Pandas: Count the first consecutive True values

Question

I am trying to implement a function that identifies the first consecutive occurrences in a Pandas Series, which has already been masked with the condition I wanted: (e.g.)

[True, True, True, False, True, False, True, True, True, True]

I want the above input to give the result of 3, i.e., there are 3 True occurrences in a row from the beginning of the series.

I am aware that a big for loop would do the work, but are there any vectorized / Pandas-centric way to go around it?

Thank you very much.

Answer 1

I initally hadn't got that the question was about pandas Series. The answer below uses list methods, but is still valid if the pandas Series are converted to list (method a.to_list()).

Without any import, you can find the first occurrence of False by using the method index of list, and this will be the number you are looking for:

a = [True, True, True, False, True, False, True, True, True, True]
n_head_true = a.index(False) if False in a else len(a)
print(n_head_true)
> 3

a = [True, True, True]
n_head_true = a.index(False) if False in a else len(a)
print(n_head_true)
> 3

a = [False, True, True]
n_head_true = a.index(False) if False in a else len(a)
print(n_head_true)
> 0

The if is needed because index gives error if all values are True.