正则表达式?

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

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

myString.count('a');

更多信息请点击这里

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.count():

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

我是pandas库的粉丝，尤其是value_counts()方法。你可以用它来计算字符串中每个字符的出现次数:

>>> import pandas as pd
>>> phrase = "I love the pandas library and its `value_counts()` method"
>>> pd.Series(list(phrase)).value_counts()
     8
a    5
e    4
t    4
o    3
n    3
s    3
d    3
l    3
u    2
i    2
r    2
v    2
`    2
h    2
p    1
b    1
I    1
m    1
(    1
y    1
_    1
)    1
c    1
dtype: int64