A Simple Interactive Loop

Suppose you’re asked to write a Python program that interacts with a user in a console window. Maybe you’re accepting inputs to send to a database, or reading numbers to be used in a calculation. Regardless of the purpose, you need to code a loop that reads one or more inputs from a user typing on a keyboard, and prints back a result for each. In other words, you need to write a classic read/evaluate/print loop program.

In Python, typical boilerplate code for such an interactive loop might look like this:

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while True:
    reply = input('Enter text:')
    if reply == 'stop': break
    print(reply.upper())

This code makes use of a few new ideas:

 

 
  • The code leverages the Python while loop, Python’s most general looping statement. We’ll study the while statement in more detail later, but in short, it consists of the word while, followed by an expression that is interpreted as a true or false result, followed by a nested block of code that is repeated while the test at the top is true (the word True here is considered always true).
  • The input built-in function we met earlier in the book is used here for general console input—it prints its optional argument string as a prompt and returns the user’s typed reply as a string.
  • A single-line if statement that makes use of the special rule for nested blocks also appears here: the body of the if appears on the header line after the colon instead of being indented on a new line underneath it. This would work either way, but as it’s coded, we’ve saved an extra line.
  • Finally, the Python break statement is used to exit the loop immediately—it simply jumps out of the loop statement altogether, and the program continues after the loop. Without this exit statement, the while would loop forever, as its test is always true.

In effect, this combination of statements essentially means “read a line from the user and print it in uppercase until the user enters the word ‘stop.’” There are other ways to code such a loop, but the form used here is very common in Python code.

Notice that all three lines nested under the while header line are indented the same amount—because they line up vertically in a column this way, they are the block of code that is associated with the while test and repeated. Either the end of the source file or a lesser-indented statement will terminate the loop body block.

When run, here is the sort of interaction we get from this code:

Enter text:spam
SPAM
Enter text:42
42
Enter text:stop

Note

Version skew note: This example is coded for Python 3.0. If you are working in Python 2.6 or earlier, the code works the same, but you should use raw_input instead of input, and you can omit the outer parentheses in print statements. In 3.0 the former was renamed, and the latter is a built-in function instead of a statement (more on prints in the next chapter).