Python Programming – Basics of Python

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Python Programming – Basics of Python

Token

A token is a string of one or more characters that is significant as a group. Consider an expression:

sum=6+2

The tokens in the above expression are given in table 2-1:

Token	Token type
Sum	Identifier
=	Assignment operator
6	Integer literal
+	Addition operator
2	Integer literal

The process of converting a sequence of characters into a sequence of tokens is called “lexical analysis”. A program or function that performs lexical analysis is called a lexical analyzer, lexer, or tokenizer. A lexer is generally combined with a parser (beyond the scope of this book), which together analyze the syntax of computer language. Python supports the following categories of tokens: NEWLINE, INDENT, DEDENT, identifiers, keywords, literals, operators, and delimiters.

Keywords

The following identifiers (as shown as output in the following code) are used as reserved words (or “keywords”) of the language, and cannot be used as ordinary identifiers.

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

One can also check if an identifier is a keyword or not using its keyword ( ) function.

>>> import keyword
>>> keyword . iskeyword ( ' hi ' )
False
>>> keyword . iskeyword ( ' print ' )
True

Delimiters

The delimiter is a character that separates and organizes items of data. An example of a delimiter is the comma character, which acts as a field delimiter in a sequence of comma-separated values. Table 2-11 provides a list of tokens that serves as delimiters in Python.

Delimiters

(

)

[

]

{

}

‘

;

//=

∧=

>>=

<<=

**=

The following example shows how the use of delimiters can affect the result.

>>> 5+6/2                                         # no delimiter used
8 . 0
>>> (5+6)/2                                      # delimiter used
5 . 5

Following are few points that a Python programmer should be aware of:

The period (.) can also occur in floating-point and imaginary literals.
The simple and augmented assignment operators, serve lexically as delimiters but also perform operations.
ASCII characters “, #, and \ have special meaning as part of other tokens or are otherwise significant to the lexical analyzer.
Whitespace is not a token but serves to delimit tokens.

Integer function

The following function operates on integers (plain and long).

int.bit_length ( )
Return the number of bits necessary to represent an integer (plain or long) in binary, excluding the sign and leading zeros.

>>> n=-37
>>> bin(n)       # bin ( ) convert' integer number to a binary string
' -0b100101 ' 
>>> n.bit_length ( )
6 
>>> n=2**31 
>>> n
2147483648L 
>>> bin(n)
'0b10000000000000000000000000000000'
>>> n.bit_length ( )
32

Float functions

Some of the functions for floating-point numbers are discussed below.

float.as_integer_ratio ( )
Return a pair of integers whose ratio is exactly equal to the original float and with a positive denominator.

>>> ( -0 .25 ) . as_integer_ratio ( )
(-1 , 4)

float.is_integer ( )
Return True if the float instance is finite with integral value, otherwise it return False.

>>> (-2 . 0) . is_integer ( )
True
>>> (3 . 2) . is_integer ( )
False

Python Programming – Basics of Python

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