I wrote a python program that gets input from an input file. It works fine for small input files since after opening the file I did this to get the data
Since readlines only takes the input data and packs it into a list, it is clear it won't work for large input files. The problem is that I need to extract all the data in the input file before I begin any operation.
I will be doing lots of looping in the program and I don't know whether opening and closing the file in a single loop would be efficient.
Please advise on the best option.

Happy times

So the question is:
How large is your data file?

A Python list will hold something like 2 trillion items, but is going to be pretty slow with a very large number or records in it. If your list is going to be 100 million records or more, then consider an SQLite database instead. If it's a paltry one million records (we now live in a gigabyte world), then there should not be a problem, but you might want to consider using a dictionary or set as they are both indexed via a hash and would be much faster on lookups.

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I realized that a python list can hold as much data as the computer memory allows. On the python interpreter I gave this lines just to verify this and then on one console I gave the top command just to monitor the memory consumption of the python interpreter
There was no error, the size of the list increased infinitely and hence the memory consumption of the python interpreter.
The reason I posted this question is that I thought it was a bug in a program I had submitted to some online judge who normally test a program with large input files.
Many thanks to all contributors!

As performance is concerned, the file read from disk will be the slowest part by far!