Variable names and keywords

The variable name is arbitrary but programmers generally choose names for their variables that are meaningful. It's easier to use meaningful names for variables for better understanding of programming code. Also a very useful tip is to document (comment) in the programming code what the variable is used for.

The length of the variable name is also arbitrary and they can contain letters and numbers but they have to begin with a letter. It’s legal to used uppercase letters, but it’s a good idea to begin variable names with lowercase letters.

The special character underscore (_) can also appear in a name. Variable names can start with an underscore, but in general they are mostly used in writing library code which is intended for others to use.

The following example is example of illegal variable name. The Python will return a syntax error.

>>>69mustang = ‘Ford Mustang’
SyntaxError: invalid syntax
>>>mail@=190
SyntaxError: invalid syntax 
>>>class = ‘Python’ 
SyntaxError: invalid syntax

69mustang is illegal variable name because it doesn’t begin with a letter. mail@ is illegal because it contains special character which is illegal for variable name in python. Class is also illegal because it is reserved by Python. In other words the class is one of the Python keywords. The interpreter uses keywords to recognize the structure of a program, and they cannot be used as variable names.

To find out which keywords are reserved by Python in Python type the following code.

>>> import keywords 
>>>keyword.kwlist 
['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']

If you type import keywords the Python interpreter will call file called “keywords.py” and if you type keyword.kwlist it will list out all the keywords used by Python.

If you’re using Spyder program for Python programming you have to type print keyword.kwlist in order to print out all the keywords used by Python programming language.

Pay attention to variable names. If the interpreter complains about one of your variable names and you don’t know why, check this list to see if you’re variable name is on this list.