Input and Output

There will be situations where your program has to interact with the user. For example, you would want to take input from the user and then print some results back. We can achieve this using theinput() function andprint function respectively.

For output, we can also use the various methods of thestr (string) class. For example, you can use therjust method to get a string which is right justified to a specified width. Seehelp(str) for more details.

Another common type of input/output is dealing with files. The ability to create, read and write files is essential to many programs and we will explore this aspect in this chapter.

Input from user

Save this program asio_input.py:

defreverse(text):return text[::-1]defis_palindrome(text):return text == reverse(text)something =input("Enter text: ")if is_palindrome(something):print("Yes, it is a palindrome")else:print("No, it is not a palindrome")

Output:

$ python3 io_input.pyEnter text: sirNo, it is not a palindrome$ python3 io_input.pyEnter text: madamYes, it is a palindrome$ python3 io_input.pyEnter text: racecarYes, it is a palindrome

How It Works

We use the slicing feature to reverse the text. We've already seen how we can makeslices from sequences using theseq[a:b] code starting from positiona to positionb. We can also provide a third argument that determines thestep by which the slicing is done. The default step is1 because of which it returns a continuous part of the text. Giving a negative step, i.e.,-1 will return the text in reverse.

Theinput() function takes a string as argument and displays it to the user. Then it waits for the user to type something and press the return key. Once the user has entered and pressed the return key, theinput() function will then return that text the user has entered.

We take that text and reverse it. If the original text and reversed text are equal, then the text is apalindrome.

Homework exercise

Checking whether a text is a palindrome should also ignore punctuation, spaces and case. For example, "Rise to vote, sir." is also a palindrome but our current program doesn't say it is. Can you improve the above program to recognize this palindrome?

If you need a hint, the idea is that...¹

Files

You can open and use files for reading or writing by creating an object of thefile class and using itsread,readline orwrite methods appropriately to read from or write to the file. The ability to read or write to the file depends on the mode you have specified for the file opening. Then finally, when you are finished with the file, you call theclose method to tell Python that we are done using the file.

Example (save asio_using_file.py):

poem ='''\Programming is funWhen the work is doneif you wanna make your work also fun:    use Python!'''# Open for 'w'ritingf =open('poem.txt','w')# Write text to filef.write(poem)# Close the filef.close()# If no mode is specified,# 'r'ead mode is assumed by defaultf =open('poem.txt')whileTrue:    line = f.readline()# Zero length indicates EOFiflen(line) ==0:break# The `line` already has a newline# at the end of each line# since it is reading from a file.print(line, end='')# close the filef.close()

Output:

$ python3 io_using_file.pyProgramming is funWhen the work is doneif you wanna make your work also fun:    use Python!

How It Works

Note that we can create a new file object simply by using theopen method. We open (or create it if it doesn't already exist) this file by using the built-inopen function and specifying the name of the file and the mode in which we want to open the file. The mode can be a read mode ('r'), write mode ('w') or append mode ('a'). We can also specify whether we are reading, writing, or appending in text mode ('t') or binary mode ('b'). There are actually many more modes available andhelp(open) will give you more details about them. By default,open() considers the file to be a 't'ext file and opens it in 'r'ead mode.

In our example, we first open/create the file in write text mode and use thewrite method of the file object to write our string variablepoem to the file and then we finallyclose the file.

Next, we open the same file again for reading. We don't need to specify a mode because 'read text file' is the default mode. We read in each line of the file using thereadline method in a loop. This method returns a complete line including the newline character at the end of the line. When anempty string is returned, it means that we have reached the end of the file and we 'break' out of the loop.

In the end, we finallyclose the file.

We can see from ourreadline output that this program has indeed written to and read from our newpoem.txt file.

Pickle

Python provides a standard module calledpickle which you can use to storeany plain Python object in a file and then get it back later. This is called storing the objectpersistently.

Example (save asio_pickle.py):

import pickle# The name of the file where we will store the objectshoplistfile ='shoplist.data'# The list of things to buyshoplist = ['apple','mango','carrot']# Write to the filef =open(shoplistfile,'wb')# Dump the object to a filepickle.dump(shoplist, f)f.close()# Destroy the shoplist variabledel shoplist# Read back from the storagef =open(shoplistfile,'rb')# Load the object from the filestoredlist = pickle.load(f)print(storedlist)f.close()

Output:

$ python io_pickle.py['apple', 'mango', 'carrot']

How It Works

To store an object in a file, we have to firstopen the file inwritebinary mode and then call thedump function of thepickle module. This process is calledpickling.

Next, we retrieve the object using theload function of thepickle module which returns the object. This process is calledunpickling.

Unicode

So far, when we have been writing and using strings, or reading and writing to a file, we have used simple English characters only. Both English and non-English characters can be represented in Unicode (please see the articles at the end of this section for more info), and Python 3 by default stores string variables (think of all that text we wrote using single or double or triple quotes) in Unicode.

NOTE: If you are using Python 2, and we want to be able to read and write other non-English languages, we need to use theunicode type, and it all starts with the characteru, e.g.u"hello world"

>>>"hello world"'hello world'>>>type("hello world")<class'str'>>>>u"hello world"'hello world'>>>type(u"hello world")<class'str'>

When data is sent over the Internet, we need to send it in bytes... something your computer easily understands. The rules for translating Unicode (which is what Python uses when it stores a string) to bytes is called encoding. A popular encoding to use is UTF-8. We can read and write in UTF-8 by using a simple keyword argument in ouropen function.

# encoding=utf-8import iof = io.open("abc.txt","wt", encoding="utf-8")f.write(u"Imagine non-English language here")f.close()text = io.open("abc.txt", encoding="utf-8").read()print(text)

How It Works

We useio.open and then use theencoding argument in the first open statement to encode the message, and then again in the second open statement when decoding the message. Note that we should only use encoding in the open statement when in text mode.

Whenever we write a program that uses Unicode literals (by putting au before the string) like we have used above, we have to make sure that Python itself is told that our program uses UTF-8, and we have to put# encoding=utf-8 comment at the top of our program.

You should learn more about this topic by reading:

• "The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets"
• Python Unicode Howto
• Pragmatic Unicode talk by Nat Batchelder

Summary

We have discussed various types of input/output, about file handling, about the pickle module and about Unicode.

Next, we will explore the concept of exceptions.

¹. Use a tuple (you can find a list ofallpunctuation marks here) to hold all the forbidden characters, then use the membership test to determine whether a character should be removed or not, i.e. forbidden = (!,?,., ...). ↩