pip install monki

def func():
    print('First')   # line 0
    print('Second')  # line 1
    print('Third')   # line 2

import monki

# Add code at beginning or end

monki.patch(func, start="print('Starting')", end="print('Ending')")
func()
    >>> 'Starting'
    >>> 'First'
    >>> 'Second'
    >>> 'Third'
    >>> 'Ending'

# Insert lines at any offset

monki.patch(func, insert_lines={1: "print('Injected line')", 2: "print('Another injection')"})
func()
    >>> 'First'
    >>> 'Injected line'
    >>> 'Second'
    >>> 'Another injection'
    >>> 'Third'

# Let's patch a loop inside the function!
# To do so we need to insert the loop and indent a line to go inside it.

monki.patch(func, insert_lines={1: "for i in range(3):"}, indent_lines=[1])
func()
    >>> 'First'
    >>> 'Second'
    >>> 'Second'
    >>> 'Second'
    >>> 'Third'