我不知道“最简单的”，但简单的理解可以做到:

>>> my_string = "Mary had a little lamb"
>>> sum(char == 'a' for char in my_string)
4

利用内置的和，生成器理解和bool是整数的子类的事实:如何乘字符等于'a'。

“不使用计数查找字符串中需要的字符”方法。

import re

def count(s, ch):

   pass

def main():

   s = raw_input ("Enter strings what you like, for example, 'welcome': ")  

   ch = raw_input ("Enter you want count characters, but best result to find one character: " )

   print ( len (re.findall ( ch, s ) ) )

main()

正则表达式?

import re
my_string = "Mary had a little lamb"
len(re.findall("a", my_string))

你可以使用循环和字典。

def count_letter(text):
    result = {}
    for letter in text:
        if letter not in result:
            result[letter] = 0
        result[letter] += 1
    return result

a = 'have a nice day'
symbol = 'abcdefghijklmnopqrstuvwxyz'
for key in symbol:
    print(key, a.count(key))

不超过这个IMHO -你可以添加上或下的方法

def count_letter_in_str(string,letter):
    return string.count(letter)