(Python)

I want to perform the pct_change() operation in pandas without using the operation.

For example I have an array a = [1, 2, 3, 4]

a.pct_change would result in [NaN, 1.0, 0.5, 0.333]

I want to get the same result without using pct_change().

I attempted doing (a[1:]/a) -1 but don't get the expected result.