I've read in "Dive into Python 3" that:
"The
readlines()
method now returns an iterator, so it is just as efficient asxreadlines()
was in Python 2".
See: Appendix A: Porting Code to Python 3 with 2to3: A.26 xreadlines() I/O method.
I'm not sure that's true because they don't mention it here: http://docs.python.org/release/3.0.1/whatsnew/3.0.html . How can I check that?
The readlines() method returns a list containing each line in the file as a list item. Use the hint parameter to limit the number of lines returned. If the total number of bytes returned exceeds the specified number, no more lines are returned.
The readlines method returns the entire contents of the entire file as a list of strings, where each item in the list is one line of the file. The readline method reads one line from the file and returns it as a string.
Python readline() method will return a line from the file when called. readlines() method will return all the lines in a file in the format of a list where each element is a line in the file.
readlines() is used to read all the lines at a single go and then return them as each line a string element in a list. This function can be used for small files, as it reads the whole file content to the memory, then split it into separate lines.
The readlines method doesn't return an iterator in Python 3, it returns a list
Help on built-in function readlines: readlines(...) Return a list of lines from the stream.
To check, just call it from an interactive session - it will return a list, rather than an iterator:
>>> type(f.readlines()) <class 'list'>
Dive into Python appears to be wrong in this case.
xreadlines
has been deprecated since Python 2.3 when file objects became their own iterators. The way to get the same efficiency as xreadlines
is instead of using
for line in f.xreadlines():
you should use simply
for line in f:
This gets you the iterator that you want, and helps to explain why readlines
didn't need to change its behaviour in Python 3 - it can still return a full list, with the line in f
idiom giving the iterative approach, and the long-deprecated xreadlines
has been removed completely.
Like this:
Python 3.1.2 (r312:79149, Mar 21 2010, 00:41:52) [MSC v.1500 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> f = open('/junk/so/foo.txt') >>> type(f.readlines()) <class 'list'> >>> help(f.readlines) Help on built-in function readlines: readlines(...) Return a list of lines from the stream. hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint. >>>
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