1.5 Itse1359-1030-variables and identifiers  (Page 2/7)

You can later retrieve the values that you have stored there by referring to the pigeonhole by its nickname ( identifier ). You can also store a different value in the pigeonhole later if you desire.

The concept of type

Strongly typed languages

One of the main differences between Python and programming languages such as Java and C++ is the concept of type .

In strongly-typed languages like Java and C++, variables not only have a name, they also have a type. The type determines the kind of data that you can store in thepigeonhole.

It is probably more correct to say that the type determines the values that you can store there and the operations (addition, subtraction, etc.) that you can perform on those values.

Python is not strongly typed

One of the characteristics that makes Python easier to use than Java is the fact that with Python you don't have to be concerned about the type of avariable. Python takes care of type issues for you behind the scenes. However that ease of use comes with some costs attached.

Declaration of variables

Another difference between Python and Java is that with Java, you must declare variables before you can use them. Declaration of variables is not required with Python.

With Python, if you need a variable, you simply come up with a name and start using it as a variable.

Dangerous curves ahead !

With this convenience comes some danger. You can only have one variable with the same name within the same scope (I will discuss scope in a future module) .

Don't use the same name for two variables

With Python, if you unintentionally use the same name for two or more variables, the first will be overwritten by the second. This can lead to program bugs that aredifficult to find and fix.

A more subtle danger

A more subtle danger is that you create a variable that you intend to use more than once and you spell it incorrectly in one of those uses. This can be anextremely difficult problem to find and fix. I will illustrate what I mean by this later with a sample program.

Rules for identifiers

The name for a variable must follow the naming rules for identifiers that you will find in the Python Language Reference -- 2.3. Identifiers and keywords .

Give me the rules in plain English

The notation used in the Python Language Reference to define the naming rules is a little complicated, so I will try to interpret it for you.

Allowable characters

I believe that the Python Language Reference -- 2.3. Identifiers and keywords is saying that identifiers must begin with either a letter or an underscore character. Following that, you can use anunlimited sequence of letters (uppercase A through Z or lowercase a through z) , numbers (0 through 9) , or underscore characters.

Note that although the underscore character is allowed, it has special meaning in Python. I recommend that you do not use the underscorecharacter for the identifiers that you create.

Case is significant

The letters can be uppercase or lowercase, and case is significant. In other words, the identifier Ax is not the same as the identifier aX .