Variable and Expressions in Python
Python’s
syntax for variables and expressions is close to what you would see in C or
Java, so you can skim this section if it starts looking familiar. However, you should
take note of Python’s loose typing (see below).
Python
understands the standard arithmetic operators, including +, -, / (division), and
* (multiplication). The Python interpreter makes a handy calculator:
>>> 8/2
4
>>> 5+4*6
29
Note
that the second example evaluates 29 (and not 54); the interpreter multiplies 4
by 6 before adding 5. Python uses operator precedence rules to decide what to
do first. You can control order explicitly by using parentheses:
>>>
(5+4)*6
54
In
practice, it’s often easiest to use parentheses (even when they aren’t
required) to make code more readable.
Variables
You
can use variables to hold values over time. For example, this code computes how
long it takes to watch every episode of Monty
Python’s Flying Circus (including
the two German episodes of Monty
Python’s Fliegende Zirkus):
>>>
NumberOfEpisodes=47
>>>
EpisodeLength=0.5
>>>
PythonMarathonLength=(NumberOfEpisodes*EpisodeLength)
>>>
PythonMarathonLength
23.5
A
variable is always a reference to a value. Variables do not have types, but
objects do. (Python is loosely typed; the same variable may refer to an integer value in the morning
and a string value in the afternoon.)
Python
does not require variable declarations. However, you cannot access a variable
until you have assigned it a value. If you try to access an undefined variable,
the interpreter will complain (the wording of the error may be different in your
version of Python):
>>>
print Scrmptious
Traceback
(most recent call last):
File
“<stdin>”, line 1, in ?
NameError:
There is no variable named ‘Scrumptious’
This
example raised an exception. In Python, most errors are represented by exception objects
that the surrounding code can handle. Chapter 5 describes Python’s exception-handling
abilities.
Python
is case-sensitive. This means that names that are capitalized differently refer
to different variables:
>>>
FavoriteColor=”blue”
>>>
favoritecolor=”yellow”
>>>
print FavoriteColor,favoritecolor
blue yellow
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