So far, we have learned how to write programs and communicate our intentions to the Central Processing Unit using conditional execution, functions, and iterations. We have learned how to create and use data structures in the Main Memory. The CPU and memory are where our software works and runs. It is where all of the “thinking” happens.
But if you recall from our hardware architecture discussions, once the power is turned off, anything stored in either the CPU or main memory is erased. So up to now, our programs have just been transient fun exercises to learn Python.
In this chapter, we start to work with Secondary Memory (or files). Secondary memory is not erased when the power is turned off. Or in the case of a USB flash drive, the data we write from our programs can be removed from the system and transported to another system.
We will primarily focus on reading and writing text files such as those we create in a text editor. Later we will see how to work with database files which are binary files, specifically designed to be read and written through database software.
When we want to read or write a file (say on your hard drive), we first must open the file. Opening the file communicates with your operating system, which knows where the data for each file is stored. When you open a file, you are asking the operating system to find the file by name and make sure the file exists. In this example, we open the file mbox.txt, which should be stored in the same folder that you are in when you start Python. You can download this file from www.py4e.com/code3/mbox.txt
>>> fhand = open('mbox.txt') >>> print(fhand) <_io.TextIOWrapper name='mbox.txt' mode='r' encoding='cp1252'>
open is successful, the operating system returns
us a file handle. The file handle is not the actual data
contained in the file, but instead it is a “handle” that we can use to
read the data. You are given a handle if the requested file exists and
you have the proper permissions to read the file.
If the file does not exist,
open will fail with a
traceback and you will not get a handle to access the contents of the
>>> fhand = open('stuff.txt') Traceback (most recent call last): File "<stdin>", line 1, in <module> FileNotFoundError: [Errno 2] No such file or directory: 'stuff.txt'
Later we will use
except to deal
more gracefully with the situation where we attempt to open a file that
does not exist.
A text file can be thought of as a sequence of lines, much like a Python string can be thought of as a sequence of characters. For example, this is a sample of a text file which records mail activity from various individuals in an open source project development team:
From [email protected] Sat Jan 5 09:14:16 2008 Return-Path: <[email protected]> Date: Sat, 5 Jan 2008 09:12:18 -0500 To: [email protected] From: [email protected] Subject: [sakai] svn commit: r39772 - content/branches/ Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=39772 ...
The entire file of mail interactions is available from
and a shortened version of the file is available from
These files are in a standard format for a file containing multiple mail messages. The lines which start with “From” separate the messages and the lines which start with “From:” are part of the messages. For more information about the mbox format, see https://en.wikipedia.org/wiki/Mbox.
To break the file into lines, there is a special character that represents the “end of the line” called the newline character.
In Python, we represent the newline character as a
backslash-n in string constants. Even though this looks like two
characters, it is actually a single character. When we look at the
variable by entering “stuff” in the interpreter, it shows us the
\n in the string, but when we use
>>> stuff = 'Hello\nWorld!' >>> stuff 'Hello\nWorld!' >>> print(stuff) Hello World! >>> stuff = 'X\nY' >>> print(stuff) X Y >>> len(stuff) 3
You can also see that the length of the string
three characters because the newline character is a single
So when we look at the lines in a file, we need to imagine that there is a special invisible character called the newline at the end of each line that marks the end of the line.
So the newline character separates the characters in the file into lines.
While the file handle does not contain the data for the
file, it is quite easy to construct a
for loop to read
through and count each of the lines in a file:
fhand = open('mbox-short.txt') count = 0 for line in fhand: count = count + 1 print('Line Count:', count) # Code: http://www.py4e.com/code3/open.py
We can use the file handle as the sequence in our
for loop simply counts the number of lines in the
file and prints them out. The rough translation of the
loop into English is, “for each line in the file represented by the file
handle, 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
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.
>>> fhand = open('mbox-short.txt') >>> inp = fhand.read() >>> print(len(inp)) 94626 >>> print(inp[:20]) From stephen.marquar
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
inp. It is a good idea to store the output of
read as a variable because each call to
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
When you are searching through data in a file, it is a very common pattern to read through a file, ignoring most of the lines and only processing lines which meet a particular condition. We can combine the pattern for reading a file with string methods to build simple search mechanisms.
For example, if we wanted to read a file and only print out lines which started with the prefix “From:”, we could use the string method startswith to select only those lines with the desired prefix:
fhand = open('mbox-short.txt') for line in fhand: if line.startswith('From:'): print(line) # Code: http://www.py4e.com/code3/search1.py
When this program runs, we get the following output:
From: [email protected] From: [email protected] From: [email protected] From: [email protected] ...
The output looks great since the only lines we are seeing are those
which start with “From:”, but why are we seeing the extra blank lines?
This is due to that invisible newline character. Each of the
lines ends with a newline, so the
We could use line slicing to print all but the last character, but a simpler approach is to use the rstrip method which strips whitespaces from the right side of a string as follows:
fhand = open('mbox-short.txt') for line in fhand: line = line.rstrip() if line.startswith('From:'): print(line) # Code: http://www.py4e.com/code3/search2.py
When this program runs, we get the following output:
From: [email protected] From: [email protected] From: [email protected] From: [email protected] From: [email protected] From: [email protected] From: [email protected] ...
As your file processing programs get more complicated, you may want
to structure your search loops using
continue. The basic
idea of the search loop is that you are looking for “interesting” lines
and effectively skipping “uninteresting” lines. And then when we find an
interesting line, we do something with that line.
We can structure the loop to follow the pattern of skipping uninteresting lines as follows:
fhand = open('mbox-short.txt') for line in fhand: line = line.rstrip() # Skip 'uninteresting lines' if not line.startswith('From:'): continue # Process our 'interesting' line print(line) # Code: http://www.py4e.com/code3/search3.py
The output of the program is the same. In English, the uninteresting
lines are those which do not start with “From:”, which we skip using
continue. For the “interesting” lines (i.e., those that
start with “From:”) we perform the processing.
We can use the
find string method to simulate a text
editor search that finds lines where the search string is anywhere in
the line. Since
find looks for an occurrence of a string
within another string and either returns the position of the string or
-1 if the string was not found, we can write the following loop to show
lines which contain the string “@uct.ac.za” (i.e., they come from the
University of Cape Town in South Africa):
fhand = open('mbox-short.txt') for line in fhand: line = line.rstrip() if line.find('@uct.ac.za') == -1: continue print(line) # Code: http://www.py4e.com/code3/search4.py
Which produces the following output:
From [email protected] Sat Jan 5 09:14:16 2008 X-Authentication-Warning: set sender to [email protected] using -f From: [email protected] Author: [email protected] From [email protected] Fri Jan 4 07:02:32 2008 X-Authentication-Warning: set sender to [email protected] using -f From: [email protected] Author: [email protected] ...
Here we also use the contracted form of the
where we put the
continue on the same line as the
if. This contracted form of the
the same as if the
continue were on the next line and
We really do not want to have to edit our Python code every time we want to process a different file. It would be more usable to ask the user to enter the file name string each time the program runs so they can use our program on different files without changing the Python code.
This is quite simple to do by reading the file name from the user
input as follows:
fname = input('Enter the file name: ') fhand = open(fname) count = 0 for line in fhand: if line.startswith('Subject:'): count = count + 1 print('There were', count, 'subject lines in', fname) # Code: http://www.py4e.com/code3/search6.py
We read the file name from the user and place it in a variable named
fname and open that file. Now we can run the program
repeatedly on different files.
python search6.py Enter the file name: mbox.txt There were 1797 subject lines in mbox.txt python search6.py Enter the file name: mbox-short.txt There were 27 subject lines in mbox-short.txt
Before peeking at the next section, take a look at the above program and ask yourself, “What could go possibly wrong here?” or “What might our friendly user do that would cause our nice little program to ungracefully exit with a traceback, making us look not-so-cool in the eyes of our users?”
I told you not to peek. This is your last chance.
What if our user types something that is not a file name?
python search6.py Enter the file name: missing.txt Traceback (most recent call last): File "search6.py", line 2, in <module> fhand = open(fname) FileNotFoundError: [Errno 2] No such file or directory: 'missing.txt' python search6.py Enter the file name: na na boo boo Traceback (most recent call last): File "search6.py", line 2, in <module> fhand = open(fname) FileNotFoundError: [Errno 2] No such file or directory: 'na na boo boo'
Do not laugh. Users will eventually do every possible thing they can do to break your programs, either mistakenly or with malicious intent. As a matter of fact, an important part of any software development team is a person or group called Quality Assurance (or QA for short) whose very job it is to do the craziest things possible in an attempt to break the software that the programmer has created.
The QA team is responsible for finding the flaws in programs before we have delivered the program to the end users who may be purchasing the software or paying our salary to write the software. So the QA team is the programmer’s best friend.
So now that we see the flaw in the program, we can elegantly fix it
except structure. We need to
assume that the
open call might fail and add recovery code
open fails as follows:
fname = input('Enter the file name: ') try: fhand = open(fname) except: print('File cannot be opened:', fname) exit() count = 0 for line in fhand: if line.startswith('Subject:'): count = count + 1 print('There were', count, 'subject lines in', fname) # Code: http://www.py4e.com/code3/search7.py
exit function terminates the program. It is a
function that we call that never returns. Now when our user (or QA team)
types in silliness or bad file names, we “catch” them and recover
python search7.py Enter the file name: mbox.txt There were 1797 subject lines in mbox.txt python search7.py Enter the file name: na na boo boo File cannot be opened: na na boo boo
open call is a good example of the proper
except in a Python program. We
use the term “Pythonic” when we are doing something the “Python way”. We
might say that the above example is the Pythonic way to open a file.
Once you become more skilled in Python, you can engage in repartee with other Python programmers to decide which of two equivalent solutions to a problem is “more Pythonic”. The goal to be “more Pythonic” captures the notion that programming is part engineering and part art. We are not always interested in just making something work, we also want our solution to be elegant and to be appreciated as elegant by our peers.
To write a file, you have to open it with mode “w” as a second parameter:
>>> fout = open('output.txt', 'w') >>> print(fout) <_io.TextIOWrapper name='output.txt' mode='w' encoding='cp1252'>
If the file already exists, opening it in write mode clears out the old data and starts fresh, so be careful! If the file doesn’t exist, a new one is created.
write method of the file handle object puts data
into the file, returning the number of characters written. The default
write mode is text for writing (and reading) strings.
>>> line1 = "This here's the wattle,\n" >>> fout.write(line1) 24
Again, the file object keeps track of where it is, so if you call
write again, it adds the new data to the end.
We must make sure to manage the ends of lines as we write to the file
by explicitly inserting the newline character when we want to end a
write method does not add the newline
>>> line2 = 'the emblem of our land.\n' >>> fout.write(line2) 24
When you are done writing, you have to close the file to make sure that the last bit of data is physically written to the disk so it will not be lost if the power goes off.
We could close the files which we open for read as well, but we can be a little sloppy if we are only opening a few files since Python makes sure that all open files are closed when the program ends. When we are writing files, we want to explicitly close the files so as to leave nothing to chance.
When you are reading and writing files, you might run into problems with whitespace. These errors can be hard to debug because spaces, tabs, and newlines are normally invisible:
>>> s = '1 2\t 3\n 4' >>> print(s) 1 2 3 4
The built-in function
repr can help. It takes any object
as an argument and returns a string representation of the object. For
strings, it represents whitespace characters with backslash
>>> print(repr(s)) '1 2\t 3\n 4'
This can be helpful for debugging.
One other problem you might run into is that different systems use
different characters to indicate the end of a line. Some systems use a
\n. Others use a return character,
\r. Some use both. If you move files between
different systems, these inconsistencies might cause problems.
For most systems, there are applications to convert from one format to another. You can find them (and read more about this issue) at https://www.wikipedia.org/wiki/Newline. Or, of course, you could write one yourself.
Exercise 1: Write a program to read through a file and print the contents of the file (line by line) all in upper case. Executing the program will look as follows:
python shout.py Enter a file name: mbox-short.txt FROM [email protected] SAT JAN 5 09:14:16 2008 RETURN-PATH: <[email protected]> RECEIVED: FROM MURDER (MAIL.UMICH.EDU [188.8.131.52]) BY FRANKENSTEIN.MAIL.UMICH.EDU (CYRUS V2.3.8) WITH LMTPA; SAT, 05 JAN 2008 09:14:16 -0500
You can download the file from www.py4e.com/code3/mbox-short.txt
Exercise 2: Write a program to prompt for a file name, and then read through the file and look for lines of the form:
When you encounter a line that starts with “X-DSPAM-Confidence:” pull apart the line to extract the floating-point number on the line. Count these lines and then compute the total of the spam confidence values from these lines. When you reach the end of the file, print out the average spam confidence.
Enter the file name: mbox.txt Average spam confidence: 0.894128046745 Enter the file name: mbox-short.txt Average spam confidence: 0.750718518519
Test your file on the mbox.txt and mbox-short.txt files.
Exercise 3: Sometimes when programmers get bored or want to have a bit of fun, they add a harmless Easter Egg to their program. Modify the program that prompts the user for the file name so that it prints a funny message when the user types in the exact file name “na na boo boo”. The program should behave normally for all other files which exist and don’t exist. Here is a sample execution of the program:
python egg.py Enter the file name: mbox.txt There were 1797 subject lines in mbox.txt python egg.py Enter the file name: missing.tyxt File cannot be opened: missing.tyxt python egg.py Enter the file name: na na boo boo NA NA BOO BOO TO YOU - You have been punk'd!
We are not encouraging you to put Easter Eggs in your programs; this is just an exercise.
If you find a mistake in this book, feel free to send me a fix using Github.