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

martineau
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harpalss
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    If you've already read the file ("After reading in a file"), you've already read the first line! (Assuming there is at least one line.) – William Pursell Dec 15 '09 at 02:50
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    Note that the question *as now worded* has a different meaning than it originally did. Some answers now look silly as they took into account the "after reading in a file" part (which has been removed). – Peter Hansen Aug 03 '16 at 12:42

8 Answers8

485

Use the .readline() method:

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

Note that 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

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

instead, to remove the newline.

Boris Verkhovskiy
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Tor Valamo
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    In Python 3 if the file is ascii or utf8 you don't have to specify the file encoding. And if it is not you should specify the encoding to codecs.open in Python 2 anyway. – Evpok Jul 19 '12 at 12:15
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    @Evpok *"In Python 3 if the file is ascii or utf8 you don't have to specify the file encoding"* - if only that were strictly true! The reality is slightly more messy; as noted in the docs, the default encoding used is platform-dependent (and may differ even on the same computer depending upon how you launch Python - for instance, I have seen code that worked at my normal shell by assuming UTF-8 later explode when run through Apache with `mod_wsgi`). – Mark Amery Oct 23 '16 at 22:49
28
infile = open('filename.txt', 'r')
firstLine = infile.readline()
Jaelebi
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    You are not closing the file. The [solution with `with`](http://stackoverflow.com/a/1904455/855050) is better. – a06e Dec 21 '15 at 18:43
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fline=open("myfile").readline().rstrip()
ghostdog74
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  • I came here looking for this. Especially since `rstrip()` removes the newline character. – Shashank Sawant Oct 27 '14 at 09:19
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    -1; this doesn't close the file, and returns an incorrect result if the first line contains any trailing whitespace besides the newline character itself. – Mark Amery Oct 23 '16 at 22:51
  • @MarkAmery: Actually, because the file handle is not assigned to a variable, it is garbage collected right away, which closes the file. (Though, of course, the accepted solution using a context manager is still much better.) – acdr Jan 19 '18 at 09:07
  • @acdr What you say is true for CPython but not for other Python implementations - see e.g. the [PyPy garbage collection docs](http://doc.pypy.org/en/latest/cpython_differences.html#differences-related-to-garbage-collection-strategies) which mention as a particular point of interest that *"files ... are not promptly closed when they go out of scope"*. For this reason, it's typically argued (e.g. at https://stackoverflow.com/a/7396043/1709587) that relying upon the behaviour you describe is bad practice. – Mark Amery Jan 25 '18 at 11:57
16

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()
vy32
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dan-gph
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11

This should do it:

f = open('myfile.txt')
first = f.readline()
Jarret Hardie
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10
first_line = next(open(filename))
hoju
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8

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...

Peter Hansen
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    If this is a big file, f.read() will try to load the entire file into memory which would not be a good idea. An alternative would be to read one character at a time until a newline or EOF is encountered – randomThought Dec 15 '09 at 03:04
  • Actually, all the other answers are better alternatives than that. Normally reading a file with readline() and friends would load entire blocks at a time, maybe 32K give or take, and search through that to find the next newline. Much faster and more efficient. My answer would be useful only if he's *already* loading the entire thing, in which case we can assume he's okay with having it all in memory. – Peter Hansen Dec 15 '09 at 03:23
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    I'm not convinced that I changed the meaning. It's pretty meaningless to ask how to "read" the first line from a file after "reading" the whole file. As such, it's pretty obvious to me that the asker's intent was *not* to assume that `.read()` must be called first. – Mark Amery Oct 17 '16 at 20:18
  • @MarkAmery, the question was poorly worded ("pretty meaningless") so you guessed at the intent ("obvious to me") and reworded it to match. My interpretation differs. Clearly until and unless harpalss clarifies, all we have are the original wording plus his acceptance of an answer (i.e. "seek(0)") which to me clearly shows that he *had* already read in the file, at least past the first line. – Peter Hansen Oct 17 '16 at 21:45
  • you should remove this answer. it is not the right way and can trick people. – thang May 25 '17 at 20:14
  • @thang, thanks for your input, but the original question was worded in such a way that my answer was a technically valid response, and one which was intended to be helpful for the situation *as described*. I did update it to indicate that the question was edited and the wording changed, and I think my clarifications serve to prevent any "tricking" of people. – Peter Hansen May 25 '17 at 22:52
  • :) ok, I am a lazy programmer. I looked up code online, then used it in a rush to try to get something running for a demo. For some reason, my code is super slow! Took some time to find out that this thing is loading a 4GB into memory before taking just the first line! Tor's answer is in fact "the right" answer. There are many ways to do things. Some are better. Some are worse. This is on the bad end. It bothers me that this is placed on the top, instead of Tor's answer. I assumed that because it is on top, it's best, so I didn't check... blah. I guess most of it is my laziness. – thang May 26 '17 at 05:35
  • By the way, the question's "Read *only* the first line of a file?", not "How do I read the entire file and take the first line?" – thang May 26 '17 at 05:39
  • @thang, no, you are incorrect. When I answered, the question was "After reading in a file, how would you tell Python to only read the first line?" I took that to mean the fellow had already read in the entire file contents and wanted to know how to extract the first line. I've been pretty clear about that, ever since the question was edited. – Peter Hansen May 26 '17 at 22:33
  • I think you interpreted the question wrong. "to only read the first line".... not "to only take the first line." It can be interpreted either way. I still think SO should drop the ranking for this answer. Can you think of a scenario where you would want to do this instead of Tor's answer? – thang May 27 '17 at 19:27
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    @thang, really, we're wasting the time of everyone still reading. You applied a downvote, that's all you really need to do. As for "interpreted either way", yes, it could be, which is why I wasn't wrong in offering an answer that chose one valid interpretation even if it wasn't likely to be the best one. Finally, yes, if all data had been read in for other reasons, but you still wanted just the first line (and you were a rookie programmer), you might ask this question and find my answer helpful. Please just let it go at that... nobody will be confused by it. – Peter Hansen May 31 '17 at 16:40
2

If you want to read file.txt

line1 helloworld

import linecache
# read first line
print(linecache.getline('file.txt'), 1)
>helloworld
lejencodes
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    The best option for me: However you have a syntax error (The bracket should be outside): The second line should be as follows: print(linecache.getline('file.txt', 1)) – Horitty TechMasters Dec 31 '21 at 23:36