Hello dear readers! Welcome back to another edition of our tutorial on Python. In this tutorial post, we are going to be discussing about the Python Syntax.
The Python language shares many things in common with Java, C and Perl. Nevertheless, there are some major differences between the languages.
The Python language shares many things in common with Java, C and Perl. Nevertheless, there are some major differences between the languages.
Our First Python Program
Let us now take a look at how to execute programs in the various modes of programming.
Script Mode Programming
Invoking the Python's interpreter with a script parameter begins an execution of the script and then continues with the execution until the script is finished. The Python interpreter will no longer be active once the script is finished.
Let us now write a simple Python program in a script. The Python files has an extension of .py. Type the following source code in the test.py file -
Let us now write a simple Python program in a script. The Python files has an extension of .py. Type the following source code in the test.py file -
print "Hello, Python!"
We assume that you have Python interpreter already set in the PATH variable. Now try running this code as follows -
$ python test.py
Output
This above code will produce the following output.
Hello, Python!
Let us try another way to execute a Python script. The following below is the modified test.py file -
#!/usr/bin/python
print "Hello, Python!"
print "Hello, Python!"
We assume that you have Python interpreter available in the usr/bin directory. Now, try to run the code as follows -
$ chmod +x test.py # This is to make file executable
$./test.py
$./test.py
Output
This will produce the following output -
Hello, Python!
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Interactive Mode Programming
Invoking the Python's interpreter without passing a script file as a parameter will display the below prompt -
$ python
Python 2.4.3 (#1, Nov 11 2010, 13:34:43)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
Python 2.4.3 (#1, Nov 11 2010, 13:34:43)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
Type the following text into the Python prompt and press Enter -
>>> print "Hello, Python!"
If you are running the new version of Python, then you will need to make use of the print statement along with parenthesis as in to print ("Hello, Python!");. However in Python version 2.3.4, this will produce the following output -
Hello, Python!
Python Identifiers
A Python identifier is a name that is used in identifying a variable, module, class or any other objects. It starts with the letter A to Z or even a to z or an underscore (_) followed by zero or more letters, underscores and digits (0 - 9).
Python does not allow punctuation characters such as %, $ and @ within identifiers. Python is a Case Sensitive language, therefore Hello and hello are two different identifiers in Python.
Following below are the naming convention for Python identifiers -
Python does not allow punctuation characters such as %, $ and @ within identifiers. Python is a Case Sensitive language, therefore Hello and hello are two different identifiers in Python.
Following below are the naming convention for Python identifiers -
- The class name starts with an uppercase letter. All other identifiers starts with a lower case letter.
- Starting an identifier with a single leading under score indicates that the identifier is private.
- When you start an identifier with 2 leading underscores, it indicates a more strongly private identifier.
- If the identifier ends with two trailing underscores, then the identifier is language defined special name.
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Python Reserved Keywords
The following table contains a lists of all the Python Keywords. These are reserved words and can not be used as constant or variable or any other identifier name. All of these reserved keywords contains only lowercase.
| and | exec | not |
| assert | finally | or |
| break | for | pass |
| class | from | |
| continue | global | raise |
| def | if | return |
| del | import | try |
| elif | in | while |
| else | is | with |
| except | lambda | yield |
Lines and Indentation
Python does not provide braces to indicate blocks of code for class and function definitions or flow control. The blocks of code are denoted by line indentation, which is strongly enforced.
The number of the spaces in the indentation variable, but all spaces within the block must be indented. For example -
The number of the spaces in the indentation variable, but all spaces within the block must be indented. For example -
if True:
print "True"
else:
print "False"
print "True"
else:
print "False"
However the following block below generates an error message -
if True:
print "Answer"
print "True"
else:
print "Answer"
print "False"
print "Answer"
print "True"
else:
print "Answer"
print "False"
Thus, in Python all the continues lines indented with same number of spaces is going to form a block. The following example below has various statement blocks -
Note - Do not try to understand the logic at this point in time. Just try to understand the various blocks even if they are without braces.
Note - Do not try to understand the logic at this point in time. Just try to understand the various blocks even if they are without braces.
#!/usr/bin/python
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
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
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Multi Line Statements
Statements in Python ends with a new line. Python, however, allows for the use of the line continuation character (\) to indicate that the line should continue.
Example
The following below is a short example -
total = item_one + \
item_two + \
item_three
item_two + \
item_three
Statements contained within the {}, [] or () brackets do not need to make use of the line continuation character. For example -
days = ['Monday', 'Tuesday', 'Wednesday',
'Thursday', 'Friday']
'Thursday', 'Friday']
Quotation in Python
Python program accepts single ('), double ("), triple ("' or """) quotes to denote string literals, provided the same type of quote begins and ends the string.
Triple quotes are used to span the string across multiple lines.
Triple quotes are used to span the string across multiple lines.
Example
Following below are legal -
word = 'word'
sentence = "This is a sentence."
paragraph = """This is a paragraph. It is made
up of multiple lines and sentences."""
sentence = "This is a sentence."
paragraph = """This is a paragraph. It is made
up of multiple lines and sentences."""
Python Comments
A hash sign (#) that's not inside a string literal begins a comment. All the characters after the # and up to the end of the physical line are part of the comment and is been ignored by the Python interpreter.
#!/usr/bin/python
# First comment
print "Hello, Python!" # second comment
# First comment
print "Hello, Python!" # second comment
This will produce the following output -
Hello, Python!
You can type in a comment on the same line after a statement or an expression -
name = "Madisetti" # This is again comment
You can comment multiple lines as follows -
# This is a comment.
# This is a comment, too.
# This is a comment, too.
# I said that already.
# This is a comment, too.
# This is a comment, too.
# I said that already.
The following triple quoted string is also ignored by the Python interpreter and can as well be used as a multi-line comment.
'''
This is a multiline
comment.
'''
This is a multiline
comment.
'''
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Using Blank Lines
A line containing only whitespace, maybe with a comment, is known as a blank line and Python totally ignores it.
In a Python's interactive interpreter session, to terminate a multiline statement, you must enter an empty physical line.
In a Python's interactive interpreter session, to terminate a multiline statement, you must enter an empty physical line.
Waiting for the User
The following line of the program displays the prompt, saying "Press the enter key to exit", and waits for the user to take action -
#!/usr/bin/python
raw_input("\n\nPress the enter key to exit.")
raw_input("\n\nPress the enter key to exit.")
Here, "\n\n" is used to create two new lines before displaying the actual line. Once the user presses the key, the program ends. This is a nice trick to keep the console window open until the user is done with an application.
Multiple Statement Groups as Suites
A group of individual statements, that makes up a single code block are suites. All those compound or complex statements, such as def, if, while, and class requires a header line and suite.
Header lines begin the statement and ends with a colon and are followed by one or more lines that makes up the suite.
Header lines begin the statement and ends with a colon and are followed by one or more lines that makes up the suite.
Example
The following below is a short example -
if expression :
suite
elif expression :
suite else :
suite
suite
elif expression :
suite else :
suite
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Multiple Statements on a Single Line
Semicolon (;) allows for multiple statements on a single line given that neither statement start a new code block. Following below is a sample snip using the semicolon -
import sys; x = 'foo'; sys.stdout.write(x + '\n')
Command Line Arguments
Many programs can run to provide you with some basic data about how they should be run. Python enables you to do this with -h-
$ 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
[ etc. ]
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
[ etc. ]
You can code your script in such a way that it should accept various options. The Python's Command Line Arguments is an advanced topic that will be discussed in our subsequent tutorials.
Alright guys! This is where we are rounding up for this tutorial. In our next tutorial, we will be discussing about the Python Data Types
Do feel free to ask your questions where necessary and i will attend to them as soon as possible. If this tutorial was helpful to you, you can use the share button to share this tutorial.
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Thanks for reading and bye for now.
Do feel free to ask your questions where necessary and i will attend to them as soon as possible. If this tutorial was helpful to you, you can use the share button to share this tutorial.
Follow us on our various social media platforms to stay updated with our latest tutorials. You can also subscribe to our newsletter in order to get our tutorials delivered directly to your emails.
Thanks for reading and bye for now.