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Text files are composed of plain text content. Text files are also known as flat files or plain files. Python provides easy support to read and access the content within the file. Text files are first opened and then the content is accessed from it in the order of lines. By default, the line numbers begin with the 0th index. There are various ways to read specific lines from a text file in python, this article is aimed at discussing them.
File in use: test.txt
Method 1: fileobject.readlines[]
A file object can be created in Python and then readlines[] method can be invoked on this object to read lines into a stream. This method is preferred when a single line or a range of lines from a file needs to be accessed simultaneously. It can be easily used to print lines from any random starting index to some ending index. It initially reads the entire content of the file and keep a copy of it in memory. The lines at the specified indices are then accessed.
Example:
Python3
file
=
open
[
'test.txt'
]
content
=
file
.readlines[]
print
[
"tenth line"
]
print
[content[
9
]]
print
[
"first three lines"
]
print
[content[
0
:
3
]]
Output
tenth line
This is line 10.
first three lines
This is line 1.This is line 2.This is line 3.
Method 2: linecache package
The linecache package can be imported in Python and then be used to extract and access specific lines in Python. The package can be used to read multiple lines simultaneously. It makes use of cache storage to perform optimization internally. This package opens the file on its own and gets to the particular line. This package has getline[] method which is used for the same.
Syntax:
getLine[txt-file, line_number]
Example:
Python3
import
linecache
particular_line
=
linecache.getline[
'test.txt'
,
4
]
print
[particular_line]
Output :
This is line 5.
Method 3: enumerate[]
The enumerate[] method is used to convert a string or a list object to a sequence of data indexed by numbers. It is then used in the listing of the data in combination with for loop. Lines at particular indexes can be accessed by specifying the index numbers required in an array.
Example:
Python3
file
=
open
[
"test.txt"
]
specified_lines
=
[
0
,
7
,
11
]
for
pos, l_num
in
enumerate
[
file
]:
if
pos
in
specified_lines:
print
[l_num]
Output
This is line 1. This is line 8. This is line 12.
For the sake of completeness, here is one more option.
Let's start with a definition from python docs:
slice An object usually containing a portion of a sequence. A slice is created using the subscript notation, [] with colons between numbers when several are given, such as in variable_name[1:3:5]. The bracket [subscript] notation uses slice objects internally [or in older versions, __getslice__[] and __setslice__[]].
Though the slice notation is not directly applicable to iterators in general, the itertools
package contains a replacement function:
from itertools import islice
# print the 100th line
with open['the_file'] as lines:
for line in islice[lines, 99, 100]:
print line
# print each third line until 100
with open['the_file'] as lines:
for line in islice[lines, 0, 100, 3]:
print line
The additional advantage of the function is that it does not read the iterator until the end. So you can do more complex things:
with open['the_file'] as lines:
# print the first 100 lines
for line in islice[lines, 100]:
print line
# then skip the next 5
for line in islice[lines, 5]:
pass
# print the rest
for line in lines:
print line
And to answer the original question:
# how to read lines #26 and #30
In [365]: list[islice[xrange[1,100], 25, 30, 4]]
Out[365]: [26, 30]