tell()

fob.tell()

In text mode file reading ( r )

fob=open('data1.txt','r')
print(fob.tell()) # 0 

Using write mode

fob=open('data3.txt','w')
fob.write("Hello")
print(fob.tell()) # 5
fob.close()

fob=open('data.txt','r')
fob.seek(5)
print(fob.tell()) # 5 

fob=open('data.txt','br')
fob.seek(-5,2)
print(fob.tell()) # 76

fob=open('data.txt','r')
print(fob.readline())
print(fob.tell())
fob.close()

This is first line

20