Determining how many times a substring occurs in a string in Python

Use str.count:

>>> nStr="000123000123"
>>> nStr.count('123')
2

A working version of your code:

nStr="000123000123"
pattern = '123'
count = 0
flag = True
start = 0

while flag:
    a = nStr.find(pattern, start)  # find() returns -1 if the word is not found, 
    #start i the starting index from the search starts(default value is 0)
    if a == -1:          #if pattern not found set flag to False
        flag = False
    else:               # if word is found increase count and set starting index to a+1
        count += 1        
        start = a + 1
print(count)