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8.4. Reading files¶
While the file handle object does not contain the actual data in the
file, it is quite easy to construct a
for loop to read
through and count each of the lines in a file:
We can use the file handle object in a
for loop simply counts the number of lines in the file
and prints them out. The rough translation of the
into English is, “for each line in the file represented by the file
handle object, add one to the
The reason that the
open function does not read the entire
file is that the file might be quite large with many gigabytes of data.
open statement takes the same amount of time regardless
of the size of the file. The
for loop actually causes the
data to be read from the file.
When the file is read using a
for loop in this manner,
Python takes care of splitting the data in the file into separate lines
using the newline character. Python reads each line through the newline
and includes the newline as the last character in the
variable for each iteration of the
for loop reads the data one line at a time, it
can efficiently read and count the lines in very large files without
running out of main memory to store the data. The above program can
count the lines in any size file using very little memory since each
line is read, counted, and then discarded.
If you know the file is relatively small compared to the size of your
main memory, you can read the whole file into one string using the
read method on the file handle.
In this example, the entire contents (all 94,626 characters) of the file
mbox-short.txt are read directly into the variable
inp. We use string slicing to print out the first 20
characters of the string data stored in
When the file is read in this manner, all the characters including all
of the lines and newline characters are one big string in the variable
inp. It is a good idea to store the output of
read as a variable
because each call to
read exhausts the resource:
>>> fhand = open('mbox-short.txt') >>> print(len(fhand.read())) 94626 >>> print(len(fhand.read())) 0
Remember that this form of the
open function should only be used
if the file data will fit comfortably in the main memory of your computer.
If the file is too large to fit in main memory, you should write your
program to read the file in chunks using a
Put the following code in order so that it uses a for each loop to read the file and print out the total number of lines.