Python script converts indentation sizes for python files

convert_indents.py

import sys

try:
    filename = sys.argv[1]
    indents = int(sys.argv[2])
except ValueError:
    raise ValueError('2nd arg must be integer')
except IndexError:
    raise IndexError('Missing args')

with open(filename, 'r') as f:
    lines = f.readlines()

open(filename, 'w').close()

get_indent = [l for l in lines if l.startswith(' ')]

size = 0
for char in get_indent[0]:
    if char == ' ':
        size += 1
    else:
        break

print('Indentation size detected: {}'.format(size))
new_lines = []

for line in lines:
    current_indentation = 0
    for char in line:
        if char == ' ':
            current_indentation += 1
        else:
            break

    if current_indentation:
        scale = current_indentation / size
        desired = int(scale * indents)

        string = ' ' * desired

        new_lines.append(string + line.strip() + '\n')
        continue

    new_lines.append(line)

with open(filename, 'a') as f:
    for line in new_lines:
        f.write(line)