doubling a number

[Gribouillis]

A faster version using Python's arbitrarily sized integers

def double(text):
    parts = text.split('.', 1)
    a, b = parts if len(parts) == 2 else (parts[0], '')
    shift = len(b)
    n = str(int(a + b) * 2)
    if shift:
        return f'{n[:-shift]}.{n[-shift:]}'
    else:
        return n

for num in ("1", "10", "12.5", "1.9", "99", "99.7899", "99.999"):
    print(f"{num}, {double(num)}")

This code assumes that the initial string doesn't have an exponential part such as 31.4e-1