Lots of other answers here, but to answer precisely the question you asked [before @MarkAmery went and edited the original question and changed the meaning]:
>>> f = open['myfile.txt']
>>> data = f.read[]
>>> # I'm assuming you had the above before asking the question
>>> first_line = data.split['\n', 1][0]
In other words, if you've already read in the file [as you said], and have a big block of data in memory, then to get the first line from it efficiently, do a split[] on the newline character, once only, and take the first element from the resulting list.
Note that this does not include
the \n
character at the end of the line, but I'm assuming you don't want it anyway [and a single-line file may not even have one]. Also note that although it's pretty short and quick, it does make a copy of the data, so for a really large blob of memory you may not consider it "efficient". As always, it depends...
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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.