basic_syntax

=Basic Syntax= The Python language has many similarities to Perl, C, and Java. However, there are some definite differences between the languages. This chapter is designed to quickly get you up to speed on the syntax that is expected in Python. First Python Program: Interactive Mode Programming: Invoking the interpreter without passing a script file as a parameter brings up the following prompt: code $ python Python 2.5 (r25:51908, Nov 6 2007, 16:54:01) [GCC 4.1.2 20070925 (Red Hat 4.1.2-27)] on linux2 Type "help", "copyright", "credits" or "license" for more info. >>> code || Type the following text to the right of the Python prompt and press the Enter key: code >>> print "Hello, Python!"; code || **NOTE:** If you are running new version of Python then you would need to use print statement with parenthisis like **print ("Hello, Python!");** This will produce following result: code Hello, Python! code || Script Mode Programming : Invoking the interpreter with a script parameter begins execution of the script and continues until the script is finished. When the script is finished, the interpreter is no longer active. Let us write a simple Python program in a script. All python files will have extension **.py**. So put the following source code in a test.py file. code #!/usr/bin/python

print "Hello, Python!"; code || Here I assumed that you have Python interpreter available in /usr/bin directory. Now try to run this program as follows: code $ chmod +x test.py    # This is to make file executable $ python test.py code || This will produce following result: code Hello, Python! code || You have seen a simple Python program in interactive as well as script mode, now lets see few basic concepts related to Python Syntax: Python Identifiers: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">A Python identifier is a name used to identify a variable, function, class, module, or other object. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores, and digits (0 to 9). <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Python does not allow punctuation characters such as @, $, and % within identifiers. Python is a case sensitive programming language. Thus **Manpower** and **manpower** are two different identifiers in Python. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Here are following identifier naming convention for Python: > > > > Reserved Words: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The following list shows the reserved words in Python. These reserved words may not be used as constant or variable or any other identifier names. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Keywords contain lowercase letters only. Lines and Indentation: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">One of the first caveats programmers encounter when learning Python is the fact that there are no braces to indicate blocks of code for class and function definitions or flow control. Blocks of code are denoted by line indentation, which is rigidly enforced. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The number of spaces in the indentation is variable, but all statements within the block must be indented the same amount. Both blocks in this example are fine: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">if True: print "True" else: print "False" code || <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">However, the second block in this example will generate an error: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">if True: print "Answer" print "True" else: print "Answer" print "False" code || <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Thus, in Python all the continous lines indented with similar number of spaces would form a block. Following is the example having various statement blocks: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">**Note:** Don't try to understand logic or different functions used. Just make sure you undertood various blocks even if they are without braces. code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python
 * <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> Class names start with an uppercase letter and all other identifiers with a lowercase letter.
 * <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> Starting an identifier with a single leading underscore indicates by convention that the identifier is meant to be private.
 * <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> Starting an identifier with two leading underscores indicates a strongly private identifier.
 * <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: left;"> If the identifier also ends with two trailing underscores, the identifier is a language-defined special name.
 * and || exec || not ||
 * assert || finally || or ||
 * break || for || pass ||
 * class || from || print ||
 * continue || global || raise ||
 * def || if || return ||
 * del || import || try ||
 * elif || in || while ||
 * else || is || with ||
 * except || lambda || yield ||

import sys

try: # open file stream file = open(file_name, "w") except IOError: print "There was an error writing to", file_name sys.exit print "Enter '", file_finish, print "' When finished" while file_text != file_finish: file_text = raw_input("Enter text: ") if file_text == file_finish: # close the file file.close break file.write(file_text) file.write("\n") file.close file_name = raw_input("Enter filename: ") if len(file_name) == 0: print "Next time please enter something" sys.exit try: file = open(file_name, "r") except IOError: print "There was an error reading file" sys.exit file_text = file.read file.close print file_text code || Multi-Line Statements: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Statements in Python typically end with a new line. Python does, however, allow the use of the line continuation character (\) to denote that the line should continue. For example: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">total = item_one + \ item_two + \ item_three code || <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Statements contained within the [], {}, or brackets do not need to use the line continuation character. For example: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'] code || Quotation in Python: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Python accepts single ('), double (") and triple (''' or """) quotes to denote string literals, as long as the same type of quote starts and ends the string. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The triple quotes can be used to span the string across multiple lines. For example, all the following are legal: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">word = 'word' sentence = "This is a sentence." paragraph = """This is a paragraph. It is made up of multiple lines and sentences.""" code || Comments in Python: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">A hash sign (#) that is not inside a string literal begins a comment. All characters after the # and up to the physical line end are part of the comment, and the Python interpreter ignores them. code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

print "Hello, Python!"; # second comment code || <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This will produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Hello, Python! code || <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">A comment may be on the same line after a statement or expression: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">name = "Madisetti" # This is again comment code || <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You can comment multiple lines as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;"># This is a comment. code || Using Blank Lines: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">A line containing only whitespace, possibly with a comment, is known as a blank line, and Python totally ignores it. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">In an interactive interpreter session, you must enter an empty physical line to terminate a multiline statement. Waiting for the User: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The following line of the program displays the prompt, Press the enter key to exit. and waits for the user to press the Enter key: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python
 * 1) First comment
 * 1) This is a comment, too.
 * 2) This is a comment, too.
 * 3) I said that already.

raw_input("\n\nPress the enter key to exit.") code || <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Here "\n\n" are being used to create two new lines before displaying the actual line. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Once the user presses the key, the program ends. This is a nice trick to keep a console window open until the user is done with an application. Multiple Statements on a Single Line: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The semicolon allows multiple statements on the single line given that neither statement starts a new code block. Here is a sample snip using the semicolon: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">import sys; x = 'foo'; sys.stdout.write(x + '\n') code || Multiple Statement Groups as Suites: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Groups of individual statements making up a single code block are called **suites** in Python. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Compound or complex statements, such as if, while, def, and class, are those which require a header line and a suite. <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Header lines begin the statement (with the keyword) and terminate with a colon and are followed by one or more lines which make up the suite. Example: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">if expression : suite elif expression : suite else : suite code || Command Line Arguments: <span style="background-color: #ffffff; display: block; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You may have seen, for instance, that many programs can be run so that they provide you with some basic information about how they should be run. Python enables you to do this with -h: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">$ python .h usage: python [option]...[-c cmd | -m mod | file | -] [arg].. Options and arguments (and corresponding environment variables): -c cmd : program passed in as string (terminates option list) -d    : debug output from parser (also PYTHONDEBUG=x) -E    : ignore environment variables (such as PYTHONPATH) -h    : print this help message and exit (also --help)

[ etc. ] code || You can also program your script in such a way that it should accept various options. <span style="background-color: #ffffff; color: #900b09; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">[|Command Line Arguments] is an advanced topic and should be studied a bit later once you have gone through rest of the Python concepts