Python Tutorial

Overview

Before You Start

This tutorial assumes:

• Python 2.3 or greater is installed on your system. ActivePython is a free distribution of the core Python language. See the Komodo Installation Guide for configuration instructions.
• You are interested in learning about Komodo functionality, including the debugger and the interactive shell.
• You are interested in Python and have some programming experience either in Python or another language.

Python Tutorial Scenario

The Python Tutorial demonstrates how to use the Komodo debugger and interactive shell to explore a Python program. In particular, this tutorial examines a Python script that preprocesses files (similar to the C preprocessor). In this tutorial you will:

1. Open the Python Tutorial Project.
2. Analyze preprocess.py the Python program included in the Tutorial Project.
3. Analyze contenttype.py the Python module included in the Tutorial Project.
4. Run the program and generate program output.
5. Debug the program using the Komodo debugger.
6. Explore Python using the Komodo interactive shell.

See Interactive Shell and Debugging Programs for more information on this Komodo functionality.

Opening the Tutorial Project

On the File menu, click Open|Project and select python_tutorial.kpf from the python_tutorials subdirectory. The location differs depending on your operating system.

Windows

<komodo-install-directory>\lib\support\samples\python_tutorials

Linux

<komodo-install-directory>/lib/support/samples/python_tutorials

Mac OS X

<User-home-directory>/Library/Application Support/Komodo/3.x/samples/python_tutorials

All files included in the tutorial project are displayed on the Projects tab in the Left Pane.

Overview of the Tutorial Files

The following components are included in the python_tutorial.kpf project file:

• preprocess.py: The main program. This Python program parses input source files and produces output filtered on a set of rules and statements embedded in the original input source.
• preprocess current file: A run command for executing preprocess.py on the file currently open in Komodo.
• contenttype.py: A Python module used by the main program (preprocess.py) to identify the language of a given file.
• content.types: A support file used by the Python module contenttype.py.
• helloworld.html and helloworld.py: Sample files to process using preprocess.py.

Open the Python Tutorial File

On the Projects tab, double-click the preprocess.py file. This file opens in the Editor Pane; a tab at the top of the pane displays the filename.

Analyzing the Python Files

This section reviews the code in preprocess.py and contenttype.py.

Analyzing preprocess.py

In this step, you will analyze the Python program preprocess.py in sections. This program is an advanced Python script that is best addressed by focusing on certain areas within the code. Be sure that line numbers are enabled in Komodo (View|View Line Numbers) and that preprocess.py is displayed in the Komodo Editor.

 
    #define <var>[=<value>]
    #undef <var>
    #if <expr>
    #elif <expr>
    #else
    #endif
    #error <error string>
  

Setting Up the preprocess.py Program

Lines 3 to 57 - Defining a Module Docstring

• help is defined in a module docstring
• docstrings are contained in triple-quoted strings (""")

Lines 59 to 65 - Importing Standard Python Modules

• Imports the following six modules:
• • os: operating system dependant helper routines
  • sys: functions for interacting with the Python interpreter
  • getopt: parses command line options
  • types: defines names for all type symbols in the standard Python interpreter
  • re: evaluates regular expressions
  • pprint: supports pretty-print output
  • logging: writes errors to a log file

Line 67 - Importing the contenttype Module

The custom contenttype module is used by the preprocess.py program and is not included in a standard Python installation.

• loads the contenttype module and imports the getContentType method

Defining an Exception Class

Lines 72 to 88 - Declaring an Exception

• PreprocessError class inherits from the Python Exception class
• an instance of the PreprocessError class is thrown by the preprocess module when an error occurs

Initializing Global Objects

Line 93 - Initializing log

• log is a global object used to log debug messages and error messages

Lines 98 to 111 - Mapping Language Comments

• _commentGroups is a mapping of file type (as returned by content.types) to opening and closing comments delimiters
• mapping is private to the preprocess.py module (_commentGroups is prefixed with an underscore to indicate that it is private to the preprocess.py module). This is a common technique used in variable, function, and class naming in Python coding).

Note that preprocessor directives recognized by the preprocess.py module are hidden in programming language-specific comments.

Defining a Private Method

Lines 116 to 123 - Expression Evaluation

• _evaluate method is private to the preprocess module
• evaluates the given expression string with the given context

Preprocessing a File

The preprocess method examines the directives in the sample source file and outputs the modified processed text.

Lines 129 to 140 - The preprocess Method Interface

The preprocess method takes three parameters as input:

• first parameter is the filename, infile
• second parameter specifies the output file (defaults to stdout); outfile=sys.stdout
• third parameter is an optional list of definitions for the preprocessor; defines={}

Lines 145 to 156 - Identifying the File Type

Examines how programming comments are delimited (started and ended) based on the type of file (for example, HTML, C++, Python).

• getContentType is called (imported earlier from the contenttype.py module) to determine the language type of the file
• file type is used to look up all comment delimiters (opening and closing language comment characters) in _commentGroups

Lines 159 to 166 - Defining Patterns for Recognized Directives

This section defines advanced regular expressions for finding preprocessor directives in the input file.

Lines 178 to 303 - Scanning the File to Generate Output

This block of code implements a basic state machine. The input file is scanned line by line looking for preprocessor directives with the patterns defined above (stmtRes). This code determines whether each line should be skipped or written to the output file.

• source file is processed
• output is generated by a state machine implemented in Python

Lines 311 to 349 - Interpreting Command Line Arguments

The main method takes the text entered at the command line and uses the getopt module to parse the data into arguments. These arguments are then passed into the "preprocess" method.

• runs when preprocess.py is executed as a program rather than loaded as a module
• parses the filename and any defines (-D) set as command line arguments
• passes all data to the preprocess method

Lines 351 to 352 - Running the Main Method

• runs the main method when preprocess.py is executed as a program

Analyzing contenttype.py

In this step, you will analyze the Python program contenttype.py in sections. This Python script is best addressed by focusing on certain areas within the code. Be sure that line numbers are enabled in Komodo (View|View Line Numbers) and that contenttype.py is displayed in the Komodo Editor Pane.

Open contenttype.py

On the Projects tab, double-click the contenttype.py file. This file opens in the Editor Pane; a tab at the top of the pane displays the filename.

Setting Up the contenttype.py Module

The contenttype.py module is used by the main program, preprocess.py, to identify what programming language a particular file is written in based on the file extension and several other tests.

Lines 16 to 19 - Importing External Modules

• imports external modules used in this file (re, os, sys, logging)
• logging is not a standard module; it is new in Python 2.3

Getting Data from content.types

Lines 29 to 31 - Finding the Helper File (content.types)

This section outlines the usage of the private _getContentTypesFile method located in the contenttype module.

• returns the complete path to the content.types file
• assumes the file is in the same directory as contenttype.py
• _getContentTypesFile is a private method that cannot be accessed from outside of the contenttype module

Lines 33 to 80 - Loading the Content Types from content.types

This section outlines the usage of the private _getContentTypesRegistry method located in the contenttype module.

• locates the content.types file and scans it to calculate three mappings to return, as follows:

file suffix -> content type (i.e. ".cpp", a C++ implementation file)
regex -> content type (i.e. ".*\.html?", an HTML file)
filename -> content type (i.e. "Makefile", a Makefile)

• _getContentTypesRegistry is a private method that cannot be accessed from outside of the contenttype module.
• • Lines 44 to 45: gets the content.types file; if none is specified in the parameter for the method, _getContentTypesFile is called to find the system default
  • Lines 47 to 49: lists the three mappings to return (empty mappings are created here)
  • Lines 51 to 79: opens and processes the content.types file on a line-by-line basis
  • • scanning of the file stops when the last line is found, line 57
    • • Lines 58 to 78: each line is parsed to determine which of the three mappings it contains
      • an entry is made in the matching one
      • commented lines (starts with #) are ignored
  • Lines 79 to 80: closes the content.types file and returns the mappings

Lines 85 to 118 - Determining a File's Content Type

This section outlines the usage of the public getContentType method located in the contenttype module.

• takes one parameter (the name of the file to determine the content)
• returns a string specifying the content type (for example,
  getContentType("my_web_page.htm") returns "HTML" )
• getContentType is the only publicly accessible method in the module
• • Line 92: _getContentTypesRegistry is called to load the content.types file and to load the mappings
  • Lines 96 to 99: filenameMap is first checked to determine if the whole filename can be used to find a match
  • Lines 101 to 109: if the filename has a suffix (contains a '.'), the suffix map is then used to find a match
  • Lines 111 to 117: each regex in the regex map is then used to determine if it matches the filename
  • Line 118: returns the content type for the file (returns an empty string if no match was found by the above three mappings)

Running the Program

This section reviews how to run the preprocess.py program using both a run command and the Komodo debugger.

Using a Run Command

To start, generate simple output by running the program with the preprocess current file run command, included in the python_tutorial.kpf project.

1. Open the Source File: On the Projects tab, double-click the helloworld.html file. The file opens in the Editor Pane.
2. Open the Run Command: On the Projects tab, double-click the preprocess current file run command. A Preprocess Current File dialog box appears.
3. Preprocess Current File: In the Preprocessor Options text area, enter:
   -D SAY_BYE
   Click OK to run the program.
4. View Output: The helloworld.html file output is displayed on the Command Output tab as follows:
   ['path_to_file\\python_tutorials\\helloworld.html']
<html>
<head> <title>Hello World</title> </head>
<body>
<p>Hello, World!</p>
</body>
</html>


Using the Debugger

Generate output by running the program through the debugger without setting any breakpoints.

1. Run the debugger: Select the preprocess.py tab in the editor. From the menu, select Debug|Go/Continue. In the Debugging Options dialog box, click OK to accept the defaults.
2. View the Debug Output Tab: Notice the messages in the bottom left corner of the Komodo screen; these inform you of the status of the debugger. When the program has finished, program output is displayed in the Bottom Pane, on the right side. If necessary, click the Debug Output tab to display it.

preprocess: error: incorrect number of arguments:
argv=['C:\\path_to_tutorial\\preprocess.py']

3. Specify a File to Process: On the Projects tab in the Left Pane, double-click the file helloworld.html. Note the preprocessor directives inside the comments (#) in this file. Select the preprocess.py tab in the editor. From the menu select Debug|Go/Continue. In the Script Arguments text box on the Debugging Options dialog box, enter helloworld.html. Click OK.

<html>
<head> <title>Hello World</title> </head>
<body>
preprocess: error: helloworld.html:5: #error: "SAY_BYE is not
defined, use '-D' option"

4. Specify an Argument with the Source File: Select Debug|Go/Continue. In the Script Arguments text box in the Debugging Options dialog box, enter the following source file and argument: -D SAY_BYE helloworld.html. Click OK.

<html>
<head> <title>Hello World</title> </head>
<body>
<p>Hello, World!</p>
</body>
</html>

5. View the Debug Output Tab: Notice the HTML output and compare the result to the actual file helloworld.html.
6. View Rendered HTML: On the right side of the Bottom Pane, click the HTML tab. The rendered HTML for the helloworld.html file is displayed in the Bottom Pane. Click the Output tab to return to the HTML code.
7. Create New File: To create a new HTML file that will later contain the HTML code in the Bottom Pane, select File|New|New File. In the New File dialog box, select the HTML Category. Click Open.
8. Save the Output: Delete the contents of the new HTML file tab in the Editor Pane, and then select the contents of the Output tab on the Bottom Pane. Copy the contents to the new HTML file tab in the Editor Pane. Select File|Save As to save the file with a unique name.
9. Specify Another Source File: Go through steps 3 to 5 using the file helloworld.py in place of helloworld.html. Notice how the output displayed is now in Python, (for example, print "Hello, World!"). This demonstrates how the preprocess.py program can be used to process files written in different language types.

Debugging the Program

In this step you will add breakpoints to the program and "debug" it. Adding breakpoints lets you run the program in sections, making it easier to watch variables and view the output as it is generated.

1. Set a breakpoint: On the preprocessor.py tab, click on the gray margin immediately to the left of the code on line 347 of the program. This sets a breakpoint, indicated by a red circle.
2. Run the debugger: Select Debug|Go/Continue. In the Script Arguments text box on the Debugging Options dialog box, enter the following source file and argument (if not there from a recent run): -D "SAY_BYE" helloworld.html. Click OK.

3. Watch the debug process: Notice that the line where the breakpoint is set (line 347) turns pink. Also, a yellow arrow appears on the breakpoint. This arrow indicates the position at which the debugger has halted.
4. View variables: On the Debug tab, click the Locals tab. If necessary, resize the pane by clicking and dragging the upper margin. On the Locals tab, notice the declared variables are assigned values. Examine the infile variable. This variable contains the name of the file specified above (helloworld.html).

5. Step In: Select Debug|Step In until the debugger stops at line 129, the preprocess method. "Step In" is a debugger command that causes the debugger to enter a function called from the current line.
6. Set another breakpoint: Click on the gray margin immediately to the left of the code in line 145 to set another breakpoint. Line 145 is where getContentType is called.
7. Run the debugger: Select Debug|Go/Continue.
8. Step Over: When line 145 is processed, the variable contentType is assigned the source file's (helloworld.html) type (HTML). "Step Over" is a debugger command that executes the current line of code. If the line of code calls a function or method, the function or method is executed in the background and the debugger pauses at the line that follows the original line.
9. View variables: On the Debug tab, click the Locals tab. Examine the contentType variable. This variable contains the type of the source file; the type is "HTML" for helloworld.html.
10. Set another breakpoint: Click on the gray margin immediately to the left of the code in line 197 to set another breakpoint. Line 197 is inside of the loop where the source file helloworld.html is being processed.
11. Run the debugger: Select Debug|Go/Continue.
12. Add Watches for Variables: On the Debug tab, click the Watch tab. Click the New button in the lower-right corner of the Debug tab. An Add Variable dialog box appears. In the Add Variable dialog box, enter lineNum in the text box. Click OK. Notice that the lineNum variable and its value are displayed in the Watch tab. The lineNum variable is the line number of the line currently being processed in the source file helloworld.html. Follow the above steps again to enter a watch for the variable line. The line variable contains the actual text of the line currently being processed.
13. Run the debugger: Select Debug|Go/Continue. Notice how the variables in the Watch tab change every time the debugger stops at the breakpoint set at line 197. Also, notice the output in the right side of the Debug tab. This output changes as new lines are displayed.
14. Disable and Delete a breakpoint: Click on the red breakpoint at line 197. The red beakpoint is now white with a red outline. This breakpoint is now disabled. Click on the disabled white breakpoint. This removes the breakpoint, but does not stop the debugger.
15. Stop the Debugger: On the Debug menu, click Stop.

Explore Python with the Interactive Shell

In this step you will use the interactive shell to explore the contenttype module. The Komodo interactive shell helps you test, debug, and examine your program. See Interactive Shell for more information.

If starting this section of the tutorial with currently open Python shells, please follow the steps below to ensure the Python shell's current directory is the Python Tutorial directory.

1. Close any Current Python Shells: Click the "X" button, located in the upper-right corner of the Shell tab, for each open Python shell.
2. Make python_tutorial.kpf the Active Project: On the Project menu, select Make Active Project|preprocess.

Start using the interactive shell with the Python Tutorial project files:

1. Start the Interactive Shell: On the Tools menu, select Interactive Shell|Start New Python Shell. A Python Shell tab is displayed in the Bottom Pane.
2. Import a Module: At the ">>>" Python prompt in the interactive shell, enter: import contenttype
   Notice that another ">>>" Python prompt appears after the import statment. This indicates that the contenttype module imported successfully.
3. Get Help for a Module: At the prompt, enter: help (contenttype)
   The help instructions embedded in the contenttype.py file are printed to the interactive shell screen. This is useful for easily accessing Python documentation without installing external help files.
4. Get Help for a Method in a Module: At the prompt, press the up arrow to redisplay previously entered commands. When help (contenttype) is redisplayed, enter .getContentType at the end of the command. The entire command is as follows:
   help (contenttype.getContentType)
   Press Enter. The help instructions for the getContentType method are printed to the shell screen. The ability to instantly access help on specific Python functions is a powerful use for the interactive shell.
5. Run a Method: At the prompt, enter: contenttype.getContentType("helloworld.html")
   Notice the output identifies the file type as HTML.
6. Run Another Method: At the prompt, enter: contenttype.getContentType("helloworld.py")
   Notice the output identifies the file type as Python.
7. Run a Final Method: At the prompt, enter: contenttype.getContentType("test.txt")
   Notice the output identifies the file type as Text. The contenttype module uses several tests to determine the data type used within a file. The test that determined that test.txt is a text file simply analyzed the file extension.

More Python Resources

ASPN, the ActiveState Programmer Network

ASPN, the ActiveState Programmer Network, a source of numerous resources for Python programmers, including:

• Free downloads of ActivePython, ActiveState's Python distribution.
• Searchable Python documentation.
• The Python Cookbook, a collaborative library of regular expressions for Python.

Tutorials and Reference Sites

There are many Python tutorials and beginner Python sites on the Internet, including:

• The Python Home Page, the home of all that is Python.
• Python DevCenter, run by O'Reilly Networks, which provides access to tips, articles and other Python related items.

Preprocessor Reference

The preprocess.py program in this tutorial is a simplified version of another Python preprocess.py script available via http://starship.python.net/crew/tmick/#preprocess. The version available on starship.python.net is an advanced portable multi-language file preprocessor.