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Safari | Python Developer's Handbook -> Input and Output





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Input and Output

Python, as any other language, provides means to get input from the user and also to display information to him.

Let's see how we can handle it.

					
>>> x = input ("type anything: ")
>>> print "You have typed ", x

				

Note that the input prompt can be anything, even an empty one.

If the user types 5, x is properly treated as a number. To make x become a string, the user must explicitly type the quotes.

To avoid this problem, you can use the raw_input function:

					
>>> x = raw_input ("type anything: ")
>>> print "You have typed ", x

				

Now, it doesn't matter whether the user types the quotes.

Note that the print command requires objects to be separated by commas:

					
>>> print "parrot", "sketch"
parrot sketch

				

Displaying Information

Let's delve a little bit deeper into this topic.

Python has three standard file objects, which are available from the sys module. The interpreter uses them to provide input and output facilities. (Refer to Chapter 3, "Python Libraries," for details and examplesthe sys module.)

They are known as sys.stdin, sys.stdout, sys.stderr

print statements are mapped to the sys.stdout. Hence, they send the textual representation of objects to the standard output stream:

						
>>>import sys
>>>sys.stdout.write("Nudge-nudge\n")
Nudge-nudge

					

Did you know that it is possible to re-map the standard output device?

Yes, that is possible.

You can run the following code to write to a file:

						
>>> sys.stdout = open("outputtest.txt", "w")
>>> print "hello"
>>> sys.stdout.close

>>> sys.stdout = sys.__stdout__
>>> sys.exit()

					

Note that sys.__stdout__ stores the original stdout.

The last line restores the sys.__stdout__ original value to such an extent that new print statements will display onscreen, instead of being sent to a file.

As additional information, this program uses sys.exit() to quit its execution (refer to Chapter 3 for details).

Starting with release 2.0, the print statement can have its output directed to a file-like object, as it is demonstrated in the following example.

						
print >> sys.stderr, "Sorry, you cannot do that!"

					

Formatting Operations

Python provides formatting operations similar to the printf() function from the C language.

Take a look at the following example:

						
>>> print "Mr. Lumberjack! do not sing!"

					

What if you don't want to hard-code the name inside the string? Compare the previous line of code against the following one:

						
>>> print "Mr. %s, do not sing!" % someone

					

Flexible, don't you think? And by the way, the order of the elements doesn't affect the final result.

Therefore, saying

						
>>> print "Mr. %s" % someone

					

is the same as saying

						
>>> print someone % "Mr. %s"

					

As a matter of fact, the following example shows how Python handles multiple format arguments. Note that you need to provide a tuple of values to fill the position indicated by the formatting operators (see Table 2.2).

						
>>> print "The %s has %i wings" % ("parrot", 2)

					

Table2.2. Formatting Operators Table
Formatting Operator Description
%d decimal integer
%i decimal integer
%u unsigned integer
%o octal integer
%x hexadecimal integer
%X hexadecimal integer (uppercase letters)
%f floating point as [-]m.dddddd
%e floating point as [-]m.ddddddexx
%E floating point as [-]m.ddddddExx
%g, %G floating point where the exponent is less than -4 or greater than the precision
%s any printable object (such as strings)
%c a single character
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