[python] Read only the first line of a file?

How would you get only the first line of a file as a string with Python?

first_line = next(open(filename))

Use the .readline() method (Python 2 docs, Python 3 docs):

with open('myfile.txt') as f:
    first_line = f.readline()

Some notes:

1. As noted in the docs, unless it is the only line in the file, the string returned from f.readline() will contain a trailing newline. You may wish to use f.readline().strip() instead to remove the newline.
2. The with statement automatically closes the file again when the block ends.
3. The with statement only works in Python 2.5 and up, and in Python 2.5 you need to use from __future__ import with_statement
4. In Python 3 you should specify the file encoding for the file you open. Read more...

To go back to the beginning of an open file and then return the first line, do this:

my_file.seek(0)
first_line = my_file.readline()

infile = open('filename.txt', 'r')
firstLine = infile.readline()

fline=open("myfile").readline().rstrip()

f1 = open("input1.txt", "r")
print(f1.readline())

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...

This should do it:

f = open('myfile.txt')
first = f.readline()