[Tutor] To get numeric prefix in a string if present

[Peter Otten]

On 04/05/2021 09:30, Manprit Singh wrote:

> st = "23fgh"
> st[:st.index(st.lstrip("0123456789"))]
> what about this ? This works well too for the said operation of getting the
> numeric prefix of a string .

What if the string contains only digits?

 >>> def numerical_prefix(s):
	return s[:s.index(s.lstrip("0123456789"))]

 >>> numerical_prefix("abc")
''
 >>> numerical_prefix("321abc")
'321'
 >>> numerical_prefix("321")  # wrong
''

Instead of using str.index() you could calculate the prefix length:

 >>> def numerical_prefix(s):
	return s[:len(s) - len(s.lstrip("0123456789"))]

 >>> numerical_prefix("abc")
''
 >>> numerical_prefix("321abc")
'321'
 >>> numerical_prefix("321")
'321'