Python lexical structure

Computer languages, like human languages, have a lexical structure. A source code of a Python program consists of tokens. Tokens are atomic code elements. In Python language, we have comments, variables, literals, operators, delimiters and keywords.

Comments

Comments are used by humans to clarify the source code. All comments in Python language follow the # character.

#!/usr/bin/python

# commemts.py
# author jan bodnar
# ZetCode 2008

def main():
    print "Comments example"

main()

Everything that follows the # character is ignored by the Python interpreter.

Variables

A variable is an identifier, which holds a value. In programming we say, that we assign a value to a variable. Technically speaking, a variable is a reference to a computer memory, where the value is stored. In Python language, a variable can hold a string, a number or various objects like a function or a class. Variables can be assigned different values over time.
Variables in python can be created from alfanumeric characters and underscore (_) character. A variable cannot begin with a number. The Python interpreter can easier distinguish between a number and a variable.

Value
value2
company_name

These were valid identifiers.

12Val
exx$
first-name

These were examples of invalid identifiers.
The variables are case sensitive. This means, that Price, price, and PRICEare three different identifiers.

#!/usr/bin/python

number = 10
Number = 11
NUMBER = 12

print number, Number, NUMBER

In our script, we assign three numeric values to three identifiers.

10 11 12

This is the output of the script.

A literal

A literal is any notation for representing a value within the Python source code. Technically, a literal will be assigned a value at compile time, while a variable will be assigned at runtime.

age = 29
nationality = "Hungarian"

Here we assign two literals to variables. Number 29 and string Hungarian are literals.

#!/usr/bin/python

# literals.py

name1 = "Jane"
age1 = 12

name2 = "Rose"
age2 = 16

"Patrick"
34

"Luke"
23

print name1, age1
print name2, age2

If we do not assign a literal to a variable, there is no way, how we can work with it. It is dropped.

$ ./literals.py 
Jane 12
Rose 16

This is the output of the literals.py script.

Operators

An operator is a symbol used to perform an action on some value.

+    -    ~    *    **    /    %
<<    >>    &    |    ^
and    or    not    in    not in 
is    is not    <    >    !=    <>
==    <=    >=

This is a list of operators available in Python language. We will talk about operators later in the tutorial.

Indentation

Indentation is used to delimit blocks in Python. Where other programming languages use curly brackets or keywords such as begin, end, Python uses white space. An increase in indentation comes after certain statements; a decrease in indentation signifies the end of the current block. The Python style guide recommends using 4 spaces per indentation level.

if age > 18:
    print "adult person"

for i in range(5):
    print i

After the if keyword a code block is expected. A new statement is started on a new line, indented with 4 space characters. The indentation for the following for keyword is decreased back to the initial one. The for keyword starts a new code block, where its statement(s) are indented.

Delimiters

A delimiter is a sequence of one or more characters used to specify the boundary between separate, independent regions in plain text or other data stream.

(       )       [       ]       {       }      
,       :       .       `       =       ;
+=      -=      *=      /=      //=     %=
<=      |=      ^=      >>=     <<=     **=
'       "       \       @

Delimiters are used in various area of the Python language. They are used to build expressions, string literals, tuples, dictionaries or lists.

Keywords

A keyword is a reserved word in the Python programming language. Keywords are used to perform a specific task in a computer program. For example, print a value to the console, do repetitive tasks or perform logical operations. A programmer cannot use a keyword as an ordinary variable.

and       del       from      not       while
as        elif      global    or        with
assert    else      if        pass      yield
break     except    import    print
class     exec      in        raise
continue  finally   is        return 
def       for       lambda    try

This is a list of Python keywords. We have dedicated a special chapter to keywords in this tutorial.
This was the Python lexical structure.