Learning Python - Mark Lutz [46]
Optionally accepts a string that will be printed as a prompt (e.g., input('Press Enter to exit'))
Returns to your script a line of text read as a string (e.g., nextinput = input())
Supports input stream redirections at the system shell level (e.g., python spam.py < input.txt), just as the print statement does for output
We’ll use input in more advanced ways later in this text; for instance, Chapter 10 will apply it in an interactive loop.
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Note
Version skew note: If you are working in Python 2.6 or earlier, use raw_input() instead of input() in this code. The former was renamed to the latter in Python 3.0. Technically, 2.6 has an input too, but it also evaluates strings as though they are program code typed into a script, and so will not work in this context (an empty string is an error). Python 3.0’s input (and 2.6’s raw_input) simply returns the entered text as a string, unevaluated. To simulate 2.6’s input in 3.0, use eval(input()).
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Other Icon-Click Limitations
Even with the input trick, clicking file icons is not without its perils. You also may not get to see Python error messages. If your script generates an error, the error message text is written to the pop-up console window—which then immediately disappears! Worse, adding an input call to your file will not help this time because your script will likely abort long before it reaches this call. In other words, you won’t be able to tell what went wrong.
Because of these limitations, it is probably best to view icon clicks as a way to launch programs after they have been debugged or have been instrumented to write their output to a file. Especially when starting out, use other techniques—such as system command lines and IDLE (discussed further in the section The IDLE User Interface)—so that you can see generated error messages and view your normal output without resorting to coding tricks. When we discuss exceptions later in this book, you’ll also learn that it is possible to intercept and recover from errors so that they do not terminate your programs. Watch for the discussion of the try statement later in this book for an alternative way to keep the console window from closing on errors.
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[6] It is also possible to completely suppress the pop-up DOS console window for clicked files on Windows. Files whose names end in a .pyw extension will display only windows constructed by your script, not the default DOS console window. .pyw files are simply .py source files that have this special operational behavior on Windows. They are mostly used for Python-coded user interfaces that build windows of their own, often in conjunction with various techniques for saving printed output and errors to files.
Module Imports and Reloads
So far, I’ve been talking about “importing modules” without really explaining what this term means. We’ll study modules and larger program architecture in depth in Part V, but because imports are also a way to launch programs, this section will introduce enough module basics to get you started.
In simple terms, every file of Python source code whose name ends in a .py extension is a module. Other files can access the items a module defines by importing that module; import operations essentially load another file and grant access to that file’s contents. The contents of a module are made available to the outside world through its attributes (a term I’ll define in the next section).
This module-based services model turns out to be the core idea behind program architecture in Python. Larger programs usually take the form of multiple module files, which import tools from other module files. One of the modules is designated as the main or top-level file, and this is the one launched to start the entire program.
We’ll delve into such architectural issues in more detail later in this book. This chapter is mostly interested in the fact that import operations run the code in a file that is being loaded as a final step. Because of this, importing a file is yet another way