CODEWITHSUNDEEP
python
yarz-tech
yarz-tech

Reputation: 274

Not understanding read command in Python

I'm trying to understand what's going on with my read function. I'm simply doing a readline of a text document I created in canopy. For some reason it only gives me w for whatever value I put in. I'm new to the world of Python so I'm sure its an easy answer! Thanks for your help!

import os

my_file = open(os.path.expanduser("~/Desktop/Python Files/Test Text.txt"),'r')

print my_file.readline(3)

my_file.close()

My Text document is below

w
o
r
d
s

Upvotes: 1

Views: 105

Answers (3)

unutbu
unutbu

Reputation: 879471

my_file.readline(3) reads up to 3 bytes from the first line. The first line contains a w and an end-of-line character.

If you want to read up to the first 3 bytes regardless of the line, use my_file.read(3). Note that end-of-line characters are included in the count.

If you want to print the first 3 lines, you could use

import os
with open(os.path.expanduser("~/Desktop/Python Files/Test Text.txt"),'r') as my_file:
    for i, line in enumerate(my_file):
        if i >= 3: break
        print(line)

or 

import itertools as IT
with open(os.path.expanduser("~/Desktop/Python Files/Test Text.txt"),'r') as my_file:
    for line in IT.islice(my_file, 3):
        print(line)

For short files you could instead use

with open(os.path.expanduser("~/Desktop/Python Files/Test Text.txt"),'r') as my_file:
    lines = my_file.readlines()
    for line in lines[:3]:
        print(line)

but note that my_file.readlines() returns a list of all the lines in the file. Since this can be very memory-intensive if the file is huge, and since it is usually possible to process a file line-by-line (which is much less memory-intensive), generally the first two methods of reading a file are preferred over the third.

Upvotes: 5

itzMEonTV
itzMEonTV

Reputation: 20339

'readline([size]) -> next line from the file, as a string.Retain newline. A non-negative size argument limits the maximum number of bytes to return (an incomplete line may be returned then).Return an empty string at EOF.

readline reads next line and so on.The argument size is for how many bytes should it read from the corresponding line.

Upvotes: 1

Blair
Blair

Reputation: 6693

Using f.readline does not give random access to the file. I think you want to read the third (or maybe fourth if you're zero-indexing) line. The argument that you're passing to f.readline is a maximum byte count to read, rather than a specific line to read.

Upvotes: 0

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