# lab03.py
# PUT YOUR NAME AND NETID HERE
# PUT THE DATE YOU LAST CHANGED THIS FILE HERE
# Skeleton by Walker M. White (wmw2), Lillian Lee (LJL2), Feb 2018


""" Contains a useful string function.

CAVEATS:

This module is distributed for the purposes of exercising testing and
debugging. Hence, be aware that, as originally written, the implementation of
the useful string function may contain conceptual errors.
"""


def replace_first(word,target,rep):
    """Returns: a copy of string `word` with the FIRST instance of string
    `target` in `word` replaced by string `rep`

    Example: replace_first('methos', 's', 'd') -> 'method'

    Precondition: `target` has length >=1 and appears in `word`

    """
    pos = word.count(target)  # where the first target starts

    before = word[:pos]  # part of word up to but not including first target

    after = word[pos+1:]  # part of word after target

    result = before + rep + after  # desired output
    return result