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
raw_input() function and print statement 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.
Save this program as io_input.py:
link:programs/io_input.py[role=include]Output:
link:programs/io_input.txt[role=include]
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 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 raw_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 and pressed the
return key, the raw_input() 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 a palindrome.
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]
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 (save as io_using_file.py):
link:programs/io_using_file.py[role=include]Output:
link:programs/io_using_file.txt[role=include]
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 specify whether we are reading, writing, or appending in text
mode ('t') or binary mode ('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.
In the end, we finally close the file.
Now, check the contents of the poem.txt file to confirm that the program has indeed written to
and read from that file.
Python provides a standard module called pickle using which you can store any plain Python
object in a file and then get it back later. This is called storing the object persistently.
Example (save as io_pickle.py):
link:programs/io_pickle.py[role=include]Output:
link:programs/io_pickle.txt[role=include]
To store an object in a file, we have to first open the file in write binary 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.
So far, when we have been writing and using strings, or reading and writing to a file, we have used
simple English characters only. If we want to be able to read and write other non-English
languages, we need to use the unicode type, and it all starts with the character u:
>>> "hello world"
'hello world'
>>> type("hello world")
<type 'str'>
>>> u"hello world"
u'hello world'
>>> type(u"hello world")
<type 'unicode'>
We use the unicode type instead of strings to make sure that we handle non-English languages in
our programs. However, when we read or write to a file or when we talk to other computers on the
Internet, we need to convert our unicode strings into a format that can be sent and received, and
that format is called "UTF-8". We can read and write in that format, using a simple keyword
argument to our standard open function:
link:programs/io_unicode.py[role=include]You can ignore the import statement for now, we’ll explore that in detail in the modules
chapter.
Whenever we write a program that uses Unicode literals 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.
We use io.open and provide the "encoding" and "decoding" argument to tell Python that we are
using unicode, and in fact, we have to pass in a string in the form of u"" to make it clear that
we are using Unicode strings.
You should learn more about this topic by reading:
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