使用数:

sentence = 'A man walked up to a door'
print(sentence.count('a'))
# 4

使用数:

sentence = 'A man walked up to a door'
print(sentence.count('a'))
# 4

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)

正则表达式?

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

myString.count('a');

更多信息请点击这里

Python 3

有两种方法可以做到这一点:

1)内置函数count()

sentence = 'Mary had a little lamb'
print(sentence.count('a'))`

2)不使用函数

sentence = 'Mary had a little lamb'    
count = 0

for i in sentence:
    if i == "a":
        count = count + 1

print(count)