file modification

f=open('yesterday','r',encoding='utf-8') 

f_new=open('yesterday.bak','w',encoding='utf-8') 

for line in f: 
    if 'When When I was little' in line: 
        line=line.replace('When I was little', 'When zhow was little') 
    f_new.write(line) 
f.close() 
f_new.close()