最简单的方法是一行代码:

'Mary had a little lamb'.count("a")

但是如果你想用这个也可以:

sentence ='Mary had a little lamb'
   count=0;
    for letter in sentence :
        if letter=="a":
            count+=1
    print (count)

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

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)

这个简单直接的函数可能会有帮助:

def check_freq(x):
    freq = {}
    for c in set(x):
       freq[c] = x.count(c)
    return freq

check_freq("abbabcbdbabdbdbabababcbcbab")
{'a': 7, 'b': 14, 'c': 3, 'd': 3}

如果需要理解:

def check_freq(x):
    return {c: x.count(c) for c in set(x)}

你可以使用.count():

>>> 'Mary had a little lamb'.count('a')
4

要获得所有字母的计数，请使用集合。计数器:

>>> from collections import Counter
>>> counter = Counter("Mary had a little lamb")
>>> counter['a']
4