Python CGI: An Interactive Instruction

The Tutorial:

Lesson 2: Variables in Python

Now that we have seen how a simple CGI program works, let's inspect how variables work in Python. All assignments have the syntax identifier = value. Identifier names can be begin with either an underscore or letter followed by any combination of letters, numbers, and underscores. In addition, names are case sensitive (i.e., name is different from Name). Variables have dynamic type in Python, so there is no need to specify types in declarations [4].

Table 2.1 shows the variable types of Python with examples of each.

Table 2.1: Built-in Object Types [2: 28]

Type Example

Numbers
Strings
Lists
Dictionaries
Tuples
Files
12345, 1.5, 2e+10, 2e-10, 2+4j
"Python", "CGI"
['this', ['is', 1], 'example']
{'Python':'good', 'CGI':'useful'}
(1, 'two', 3, 'IV')
input_file = open('data.txt','r').read()

The code examples below demonstrate various assignment methods and results.

Code Example 2.1:

#!/usr/bin/python

# Print the required header that tells the browser how to render the text.
print "Content-Type: text/plain\n\n"

# Initialize a few variables.
first = 5
middle = 7.5
last = 10

# Print the values.
print "first:", first
print "middle:", middle
print "last:", last

Multiple assignments can be done at once. Python handles a = b = c assignments in a right associative manner. In addition, Python performs a swap operation on the assignment:

identifier_1, identifier_2 = identifier_2, identifier_1

Code Example 2.2:

#!/usr/bin/python

# Print the required header that tells the browser how to render the text.
print "Content-Type: text/plain\n\n"

# Initialize a few variables.
first, middle, last = 5, 7.5, 10

# Print the values.
print "first:", first
print "middle:", middle
print "last:", last, "\n"

# Swap the values.
first, last = last, first

# Print the values.
print "first:", first
print "last:", last, "\n"

# Right associative assignment.
first = last = middle

# Print the values.
print "first:", first
print "middle:", middle
print "last:", last

Python contains various operators for number, string, and list operand types. Table 2.2 shows the most common.

Table 2.2: Python Operators [2: 30]

Operator Description

or
and
not
==, !=, <, <=, >, >=, <>
in, not in
+, -
*, /
%
a[i], a[i:n], a.b Logical OR
Logical AND
Logical NOT
Comparison
Sequence Membership
Addition/String Concatenation, Subraction
Multiplication/String Repetition, Division
Modulus
Indexing, Slicing, Qualification

Code Example 2.3 demonstrates the functionality of a few of the operators above. Pay close attention to the dynamic typing as a result of integer and decimal value calculations. Note that specifying a negative index works like counting back from the end of the string.

Code Example 2.3:

#!/usr/bin/python

# Print the required header that tells the browser how to render the text.
print "Content-Type: text/plain\n\n"

# Integer division resulting in a truncated value.
a = 5/2

# Print the value.
print "5/2 =", a

# Decimal division.
a = 5/2.0

# Print the value.
print "5/2.0 =", a, "\n"

# String addition and multiplication.
python_words = "python is" + 3*" really" + " neat."

# Print the string.
print python_words, "\n"

# Print parts of the string.
print "The first character is:", python_words[0]
print "The last character is:", python_words[-1]
print "In the middle is: \n", python_words[1:-1]

So far, we have covered numbers and strings fairly well. As we use more advanced features of the Python language, we will demonstrate the capabilities of lists, dictionaries, tuples, and files.

To Lesson 3 ...