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Program Execution - Python

Python is a language and an interpreter that executes other programs. Get a quick look at program execution, how to launch code and how Python runs it. This chapter is from Learning Python, second edition, by Mark Lutz and David Ascher (ISBN: 0-596-00281-5, O'Reilly, 2003).

TABLE OF CONTENTS:
  1. How Python Runs Programs
  2. Program Execution
  3. Python Virtual Machine (PVM)
  4. Execution Model Variations
  5. The Psyco Just-in-Time Compiler
  6. Future Possibilities?
By: O'Reilly Media
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July 06, 2004

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What it means to write and run a Python script depends on whether you look at these tasks as a programmer or as a Python interpreter. Both views offer important perspective on Python programming.

The Programmer’s View

In its simplest form, a Python program is just a text file containing Python statements. For example, the following file, named script1.py, is one of the simplest Python scripts we could dream up, but it passes for an official Python program:

print 'hello world'
print 2 ** 100

This file contains two Python print statements, which simply print a string (the text in quotes) and a numeric expression result (2 to the power 100) to the output stream. Don’t worry about the syntax of this code yet—for this chapter, we’re interested only in getting it to run. We’ll explain the print statement, and why you can raise 2 to the power 100 in Python without overflowing, in later parts of this book.

You can create such a file of statements with any text editor you like. By convention, Python program files are given names that end in “.py”; technically, this naming scheme is required only for files that are “imported,” as shown later in this book, but most Python files have .py names for consistency.

After you’ve typed these statements into a text file in one way or another, you must tell Python to execute the file—which simply means to run all the statements from top to bottom in the file, one after another. Python program files may be launched by command lines, by clicking their icons, and with other standard techniques. We’ll demonstrate how to invoke this execution in the next chapter. If all goes well, you’ll see the results of the two print statements show up somewhere on your computer— by default, usually in the same window you were in when you ran the program:

hello world
1267650600228229401496703205376

For example, here’s how this script ran from a DOS command line on a Windows laptop, to make sure it didn’t have any silly typos:

D:\temp> python script1.p y
hello world
1267650600228229401496703205376

We’ve just run a Python script that prints a string and a number. We probably won’t win any programming awards with this code, but it’s enough to capture the basics of program execution.

Python’s View

The brief description of the prior section is fairly standard for scripting languages, and is usually all that most Python programmers need to know. You type code into text files, and run those files through the interpreter. Under the hood, though, a bit more happens when you tell Python to “go.” Although knowledge of Python internals is not strictly required for Python programming, a basic understanding of the runtime structure of Python can help you grasp the bigger picture of program execution.

When you instruct Python to run your script, there are a few steps that Python carries out before your code actually starts crunching away. Specifically, it’s first compiled to something called byte code, and then routed to something called a virtual machine.

Byte code compilation

Internally, and almost completely hidden from you, Python first compiles your source code (the statements in your file) into a format known as byte code. Compilation is simply a translation step, and byte code is a lower-level, and platform-independent, representation of your source code. Roughly, each of your source statements is translated into a group of byte code instructions. This byte code translation is performed to speed execution—byte code can be run much quicker than the original source code statements.

You’ll notice the prior paragraph said that this is almost completely hidden from you. If the Python process has write-access on your machine, it will store the byte code of your program in files that end with a .pyc extension (“.pyc” means compiled “.py” source). You will see these files show up on your computer after you’ve run a few programs. Python saves byte code like this as a startup speed optimization. The next time you run your program, Python will load the .pyc and skip the compilation step, as long as you haven’t changed your source code since the byte code was saved. Python automatically checks the time stamps of source and byte code files to know when it must recompile.

If Python cannot write the byte code files to your machine, your program still works—the byte code is generated in memory and simply discarded on program exit.* However, because .pyc files speed startup time, you’ll want to make sure they are written for larger programs. Byte code files are also one way to ship Python pro-grams—Python is happy to run a program if all it can find are .pyc files, even if the original .py source files are absent. (See the section “Frozen Binaries” later in this chapter for another shipping option.)

* And strictly speaking, byte code is saved only for files that are imported, not for the top-level file of a program. We’ll explore imports in Chapter 3, and again in Part V. Byte code is also never saved for code typed at the interactive prompt, which is described in Chapter 3.

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