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How to get line count cheaply in Python?
Good day. i have some code below, which implements per line file reading and counter iteration.
def __set_quantity_filled_lines_in_file(self): count = 0 with open(self.filename, 'r') as f: for line in f: count += 1 return count My question is, are there methods to determine how many lines of text data in current file without per line iteration?
Thanks!
In general it's not possible to do better than reading every character in the file and counting newline characters.
It may be possible if you know details about the internal structure of the file. For example, if the file is 1024kB long, and every line is 1kB in length, then you can deduce there are 1024 lines in the file.
I'm not sure if Python has that function or not, highly doubt it, but it would essentially require reading the whole file. A newline is signified by the \n character (actually system dependent) so there is no way to know how many of those exist in a file without going through the whole file.
You could use the readlines() file method and this is probably the easiest.
If you want to be different, you could use the read() member function to get the entire file and count CR, LF,CRLR LFCR character combinations using collections.Counter class.
However, you will have to deal with the various ways of terminating lines.
Something like:
import collections f=open("myfile","rb") d=f.read() f.close() c=collections.Counter(d) lines1=c['\r\n'] lines2=c['\n\r'] lines3=c['\r']-lines1-lines2 lines4=c['\n']-lines1-lines2 nlines=lines3+lines4 
No, such information can only be retrieved by iterating over the whole file's content (or reading the whole file into memory. But unless you know for sure that the files will always be small better don't even think about doing this).
Even if you do not loop over the file contents, the functions you call do. For example, len(f.readlines()) will read the whole file into a list just to count the number of elements. That's horribly inefficient since you don't need to store the file contents at all.
\r\n in a file in less than O(n) time? If so, please provide details.
Counter() do other than iterate over the file's content? And what other does your f.read() do than reading the whole file content, needing an unnecessary amount of memory?
len(r.readlines()) does it without iterating manually but the whole file is read into a list and then thrown away after determining its length. So it's a waste of memory (although that only applies for a rather short time)