Python Keywords and Identifiers

Python Keywords and Identifiers#

You will learn about keywords (reserved words in Python) and identifiers (names given to variables, functions, etc.).

Keywords#

Keywords are the reserved words in Python.

  1. We cannot use a keyword as a variable name, function name or any other identifier.

  2. In Python, keywords are case sensitive.

All the keywords except True, False and None are in lowercase and they must be written as they are. The list of all the keywords is given below.

Keywords in Python

False

break

for

not

None

class

from

or

True

continue

global

pass

__peg_parser__

def

if

raise

and

del

import

return

as

elif

in

try

assert

else

is

while

async

except

lambda

with

await

finally

nonlocal

yield

You can see this list any time by typing help keywords to the Python interpreter.

Trying to create a variable with the same name as any reserved word results in an error:

>>>for = 6

File "<ipython-input-1-50b154750974>", line 1
for = 6 # It will give error becasue "for" is keyword and we cannot use as a variable name.
        ^
SyntaxError: invalid syntax

for = 6 # It will give error becasue "for" is keyword and we cannot use as a variable name.

  File "<ipython-input-1-473137c03db7>", line 1
    for = 6 # It will give error becasue "for" is keyword and we cannot use as a variable name.
        ^
SyntaxError: invalid syntax

For = 6 # "for" is keyword but "For" is not keyword so we can use it as variable name
For

6

Identifiers#

An identifier is a name given to entities like class, functions, variables, etc. It helps to differentiate one entity from another.

Rules for writing identifiers#

  1. Identifiers can be a combination of letters in lowercase (a to z) or uppercase (A to Z) or digits (0 to 9) or an underscore _. Names like myClass, var_1 and print_this_to_screen, all are valid example.

  2. An identifier cannot start with a digit. 1variable is invalid, but variable1 is perfectly fine.

  3. Keywords cannot be used as identifiers

>>>global = 3

File "<ipython-input-2-43186c7d3555>", line 1
    global = 3 # because "global" is a keyword
           ^
SyntaxError: invalid syntax

global = 3 # because "global" is a keyword

  File "<ipython-input-3-3e859d37287a>", line 1
    global = 3 # because "global" is a keyword
           ^
SyntaxError: invalid syntax

  1. We cannot use special symbols like !, @, #, $, % , etc. in our identifier.

>>>m@ = 3

File "<ipython-input-3-4d4a0e714c73>", line 1
    m@ = 3
       ^
SyntaxError: invalid syntax

m@ = 3

  File "<ipython-input-4-0f73a35e8ce2>", line 1
    m@ = 3
       ^
SyntaxError: invalid syntax

NOTE

Python is a case-sensitive language. This means, Variable and variable are not the same.

Always give the identifiers a name that makes sense. While c = 10 is a valid name, writing count = 10 would make more sense, and it would be easier to figure out what it represents when you look at your code after a long gap.

Multiple words can be separated using an underscore, like this_is_a_long_variable.

this_is_a_long_variable = 6+3
this_is_a_long_variable

9

add_6_and_3 = 6+3
add_6_and_3

9
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