[Tutor] Searching backwards from end of string

[Angus Rodgers]

I'm probably having a bit of an off-day, so please forgive me if
this is a really silly question (even for a beginner in Python)!

I'm working through the exercises for Chapter 6 of Wesley Chun's
book [incidentally, I tried to append another endorsement to the
list, but my use of Python rather than "+1" may have looked like
a typo or the work of a lunatic, or perhaps my post is just late
in appearing - anyway, another happy reader here!], and running
again into a difficulty which I "cheated" my way through earlier.

On the earlier occasion:

 "6-6 Strings. Create the equivalent of string.strip():  Take a
 string and remove all leading and trailing whitespace. (Use of
 string.*strip() defeats the purpose of this exercise.)"

I wrote:

from string import whitespace

def lstrip(s):
    if s:
        for i, c in enumerate(s):
            if c not in whitespace:
                break
        return s[i:]
    else:
        return s

def rstrip(s):
    return lstrip(s[::-1])[::-1]
    # Note: far from maximally efficient (two unnecessary copies!)

def strip(s):
    return lstrip(rstrip(s))

On the present occasion:

 "6-12 Strings. ...
 (b) Create another function called rfindchr() that will find the
 last occurrence of a character in a string.  Naturally this works
 similarly to findchr(), but it starts its search from the end of
 the input string.
 ..."

I thought I had better err in the opposite direction, so I wrote:

def findchr(strng, ch):
    for i, c in enumerate(strng):
        if c == ch:
            return i
    return -1

def rfindchr(strng, ch):
    # Surely there's a neater (but still efficient) way?
    n = len(strng)
    for i in range(n - 1, -1, -1):
        if strng[i] == ch:
            return i
    return -1

def subchr(strng, origchar, newchar):
    chars = list(strng)
    for i, c in enumerate(chars):
        if c == origchar:
            chars[i] = newchar
    return ''.join(chars)

I've quoted all my code, in case it's all bad, but it's rfindchr()
that I'm particularly worried about.
-- 
Angus Rodgers