Lookahead while doing: for line in fh.readlines():

[Veek. M]

Terry Reedy wrote:

> On 2/27/2016 4:39 AM, Veek. M wrote:
>> I want to do something like:
>>
>> #!/usr/bin/env python3
>>
>> fh = open('/etc/motd')
>> for line in fh.readlines():
>>      print(fh.tell())
>>
>> why doesn't this work as expected.. fh.readlines() should return a
>> generator object and fh.tell() ought to start at 0 first.
> 
> Not after you have already read some data.  Readlines() reads the
> entire
> file and splits it into lines.  readline reads at least a single
> block.
>   Reading a single byte or character at a time looking for /n would be
> too slow, so even after readline, the file pointer will be somewhere
> past the end of the last line returned.
> 
>> Instead i get the final count repeated for the number of lines.
>>
>> What i'm trying to do is lookahead:
>> #!whatever
>>
>> fh = open(whatever)
>> for line in fh.readlines():
>>      x = fh.tell()
>>      temp = fh.readline()
>>      fh.seek(x)
>>
> 
> 

I get that readlines() would slurp the whole file for efficiency 
reasons. Why doesn't fh.seek() work though. Object 'fh' is a data 
structure for the OS file descriptor similar to FILE in C.
<class '_io.TextIOWrapper'>

So if seek works in C, how come it doesn't work in python wrt 
readlines() which is just a method. What obviates the functioning of 
seek wrt readlines()?

fh.tell() works at the line level.. and fh.readline() works with 
fh.seek(0)