Conter frecuencia de los caracteres en la cadena

#count the frequency of a character in a string using a dictionary
s=input("enter string: ")
a={}
for i in s:
    if i not in a:
        a[i]=s.count(i)
    else:
        pass
print("frequency")
for ch in a:
    print(ch,a[ch])
#output
enter string: hello world
frequency
h 1
e 1
l 3
o 2
  1
w 1
r 1
d 1
Gr@Y_orphan_ViLL@in##