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Python en:Input Output
Introduction
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 the input() and print() functions respectively.
For output, we can also use the various methods of the str (string) class. For example, you
can use the rjust method to get a string which is right justified to a specified width. See
help(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
#!/usr/bin/python
# user_input.py
def reverse(text):
return text[::-1]
def is_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:
$ python user_input.py
Enter text: sir
No, it is not a palindrome
$ python user_input.py
Enter text: madam
Yes, it is a palindrome
$ python user_input.py
Enter text: racecar
Yes, it is a palindrome
How It Works:
We use the slicing feature to reverse the text. We've already seen how we can make slices
from sequences using the seq[a:b] code starting from position a to position b. We can
also provide a third argument that determines the step by which the slicing is done. The
Python en:Input Output 91
default step is 1 because of which it returns a continuous part of the text. Giving a negative
step, i.e., -1 will return the text in reverse.
The input() 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, the
input() function will then return that text.
We take that text and reverse it. If the original text and reversed text are equal, then the
text is a palindrome (http:/ / en. wiktionary. org/ wiki/ palindrome).
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?
Files
You can open and use files for reading or writing by creating an object of the file class
and using its read, readline or write 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 the close method to
tell Python that we are done using the file.
Example:
#!/usr/bin/python
# Filename: using_file.py
poem = '''\
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!
'''
f = open('poem.txt', 'w') # open for 'w'riting
f.write(poem) # write text to file
f.close() # close the file
f = open('poem.txt') # if no mode is specified, 'r'ead mode is assumed
by default
while True:
line = f.readline()
if len(line) == 0: # Zero length indicates EOF
break
print(line, end='')
f.close() # close the file
Output:
$ python using_file.py
Programming is fun
Python en:Input Output 92
When the work is done
if you wanna make your work also fun:
use Python!
How It Works:
First, open a file by using the built-in open 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 by dealing with a text file ('t') or a binary
file ('b'). There are actually many more modes available and help(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 the file in write text mode and use the write method of the
file object to write to the file and then we finally close 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 the readline
method in a loop. This method returns a complete line including the newline character at
the end of the line. When an empty string is returned, it means that we have reached the
end of the file and we 'break' out of the loop.
By deafult, the print() function prints the text as well as an automatic newline to the
screen. We are suppressing the newline by specifying end='' because the line that is read
from the file already ends with a newline character. Then, we finally close the file.
Now, check the contents of the poem.txt file to confirm that the program has indeed
written and read from that file.
Pickle
Python provides a standard module called pickle using which you can store any Python
object in a file and then get it back later. This is called storing the object persistently.
Example:
#!/usr/bin/python
# Filename: pickling.py
import pickle
# the name of the file where we will store the object
shoplistfile = 'shoplist.data'
# the list of things to buy
shoplist = ['apple', 'mango', 'carrot']
# Write to the file
f = open(shoplistfile, 'wb')
pickle.dump(shoplist, f) # dump the object to a file
f.close()
del shoplist # destroy the shoplist variable
# Read back from the storage
Python en:Input Output 93
f = open(shoplistfile, 'rb')
storedlist = pickle.load(f) # load the object from the file
print(storedlist)
Output:
$ python pickling.py
['apple', 'mango', 'carrot']
How It Works:
To store an object in a file, we have to first open the file in 'w'rite 'b'inary mode and then
call the dump function of the pickle module. This process is called pickling.
Next, we retrieve the object using the load function of the pickle module which returns
the object. This process is called unpickling.
Summary
We have discussed various types of input/output and also file handling and using the pickle
module.
Next, we will explore the concept of exceptions.