In this part of the Python programming tutorial, we cover keywords in Python language.
Python keyword is a special word that forms the vocabulary of the Python language. It is a reserved word that cannot be used as an identifier.
Python keywords list
The following is a list of keywords for the Python programming language.
False def if raise None del import return True elif in try and else is while as except lambda with assert finally nonlocal yield break for not class from or continue global pass
Python is a dynamic language. It changes during time. The list of keywords may change in the future.
#!/usr/bin/python3 # keywords.py import sys import keyword print("Python version: ", sys.version_info) print("Python keywords: ", keyword.kwlist)
This script prints the version of Python and its actual keyword list.
$ ./keywords.py Python version: sys.version_info(major=3, minor=5, micro=2, releaselevel='final', serial=0) Python keywords: ['False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield']
The output show the list of Python keywords for Python 3.5.2.
Python control flow
while keyword is a basic keyword for controlling the
flow of the program. The statements inside the while loop are executed
until the expression evaluates to
#!/usr/bin/python3 # while_kwd.py numbers = [22, 34, 12, 32, 4] sum = 0 i = len(numbers) while (i != 0): i -= 1 sum = sum + numbers[i] print("The sum is:", sum)
In our script we want to calculate the sum of all values in the numbers list.
We utilize the
while loop. We determine the length of the list.
while loop is executed over and over again, until the
is equal to zero. In the body of the loop, we decrement the counter and calculate
the sum of values.
$ ./while_kwd.py The sum is: 104
The sum of values is 104.
break keyword is used to interrupt the cycle if needed.
#!/usr/bin/python3 # break_kwd.py import random while (True): val = random.randint(1, 30) print (val, end=" ") if (val == 22): break print()
In our example, we print random integer numbers. If the number equals to 22,
the cycle is interrupted with the
$ ./break_kwd.py 14 14 30 16 16 20 23 15 17 22
The next example shows the
continue keyword. It is used
to interrupt the current cycle, without jumping out of the whole cycle.
It initiates a new cycle.
#!/usr/bin/python3 # continue_kwd.py import random num = 0 while (num < 1000): num = num + 1 if (num % 2) == 0: continue print(num, end=" ")
In the example we print all numbers smaller than 1000 that cannot be divided by number 2 without a remainder.
if keyword is a common control flow keyword. It is used to
determine which statements are going to be executed.
#!/usr/bin/python3 # if_kwd.py age = 17 if age > 18: print("Driving licence issued") else: print("Driving licence not permitted")
if keyword tests if the the person is older than 18.
If the condition is met, the driving license is issued. The
keyword is optional. The statement after the else keyword is executed,
unless the condition is
Next we see how we can combine the statements using the
#!/usr/bin/python3 # elif_kwd.py name = "Luke" if name == "Jack": print("Hello Jack!") elif name == "John": print("Hello John!") elif name == "Luke": print("Hello Luke!") else: print("Hello there!")
If the first test evaluates to
False, we continue with the next one.
If none of the tests is
True, the else statement is executed.
$ ./elif_kwd.py Hello Luke!
This is the output.
for keyword is used to iterate over items of a
collection in order that they appear in the container.
#!/usr/bin/python3 # for_kwd.py lyrics = """\ Are you really here or am I dreaming I can't tell dreams from truth for it's been so long since I have seen you I can hardly remember your face anymore """ for i in lyrics: print(i, end=" ")
In the example, we have a
lyrics variable having a strophe of a song.
We iterate over the text and print the text character by character.
The comma in the
$ ./for_kwd.py A r e y o u r e a l l y h e r e o r a m I d r e a m i n g I c a n ' t t e l l d r e a m s f r o m t r u t h f o r i t ' s b e e n s o l o n g s i n c e I h a v e s e e n y o u I c a n h a r d l y r e m e m b e r y o u r f a c e a n y m o r e
This is the output of the script.
Python Boolean expressions
First we introduce keywords that work with boolean values and expressions:
#!/usr/bin/python3 # objects.py print(None == None) print(None is None) print(True is True) print( == ) print( is ) print("Python" is "Python")
== operator tests for equality. The
tests for object identity. Whether we are talking about the same object.
Note that multiple variables may refer to the same object.
$ ./objects.py True True True True False True
The output might be surprising for you. In Python language, there is only
None and one
True object. That is why
is equal and also identical to
True. There is only one truth out there, anyway.
The empty list
 is equal to another empty list
; but they are
not identical. Python has put them into two different memory locations. They are two distinct objects.
Hence the is keyword returns
False. On the other hand,
"Python" is "Python"
True. This is because of optimization. If two string literals are
equal, they have been put to same memory location. A string is an immutable
entity and therefore, no harm can be done.
not keyword negates a boolean value.
#!/usr/bin/python3 # not_kwd.py grades = ["A", "B", "C", "D", "E", "F"] grade = "L" if grade not in grades: print("unknown grade")
In our example we test, whether the grade value is from the list of possible grades.
$ ./not_kwd.py unknown grade
and is used if all conditions in a
boolean expression must be met.
#!/usr/bin/python3 # and_kwd.py sex = "M" age = 26 if age < 55 and sex == "M": print("a young male")
In our example, we test if two conditions are met. The
"a young male" string
is printed to the console if variable
age is less than
sex is equal to
$ ./and_kwd.py a young male
or is used if at least one condition must be met.
#!/usr/bin/python3 # or_kwd.py name = "Jack" if (name == "Robert" or name == "Frank" or name == "Jack" or name == "George" or name == "Luke"): print("This is a male")
If at least one of the expressions is true, the print statement is executed.
When we work with and/or keywords in Python programming language, short circuit evaluation takes place. Short circuit evaluation means that the second argument is only evaluated if the first argument does not suffice to determine the value of the expression: when the first argument of and evaluates to false, the overall value must be false; and when the first argument of or evaluates to true, the overall value must be true.
A typical example follows.
#!/usr/bin/python3 # short_circuit.py x = 10 y = 0 if (y != 0 and x/y < 100): print("a small value")
The first part of the expression evaluates to false. The second part of
the expression is not evaluated. Otherwise, we would get a
The following keywords are used with modules. Modules are files in which we organize our Python code.
import keyword is used to import other modules
into a Python script.
#!/usr/bin/python3 # import_kwd.py import math print(math.pi)
We use the
import keyword to import the math module into the namespace of
our script. We print the PI value.
We use the
as keyword if we want to give a module a
#!/usr/bin/python3 # as_kwd.py import random as rnd for i in range(10): print (rnd.randint(1, 10), end=" ") print()
In this case, we import the random module. We print ten random
integer numbers. We give the random module a different alias, namely
rnd. In the script we reference the module with the
$ ./as_kwd.py 1 2 5 10 10 8 2 9 7 2
from keyword is used for importing a specific
variable, class, or a function from a module.
#!/usr/bin/python3 # from_kwd.py from sys import version print(version)
sys module, we import the
version variable. If we
want to print it, we do not need to use the module name. The version
variable was imported directly to our namespace and we can reference it directly.
$ ./from_kwd.py 3.5.2 (default, Nov 17 2016, 17:05:23) [GCC 5.4.0 20160609]
Here we describe keywords associated with functions.
def keyword is used to create a new user defined
function. Functions are objects in which we organize our code.
#!/usr/bin/python3 # def_kwd.py def root(x): return x * x a = root(2) b = root(15) print(a, b)
The example demonstrates a new simple function. The function calculates
the square of a number. The
return key is closely
connected with a function definition; it exits the
function and returns a value. The value is then assigned to the
lambda keyword creates a new anonymous function.
An anonymous function is not bound to a specific name.
#!/usr/bin/python3 # lambda_kwd.py a = lambda x: x * x for i in (1, 2, 3, 4, 5): print(a(i), end=" ") print()
As you can see in the previous example, we do not create a new
function with a
def keyword. Instead of that we
use an inline function on the fly.
$ ./lambda_kwd.py 1 4 9 16 25
If we want to access variables defined outside functions, we use the
#!/usr/bin/python3 # global_kwd.py x = 15 def function(): global x x = 45 function() print(x)
Normally, assigning to
x variable inside a function, we create a new local
variable, which is valid only in that function. But if we use the global
keyword, we change a variable ouside the function definition.
$ ./global_kwd.py 45
Next we work with keywords that are used with exception handling.
$ cat films Fargo Aguirre, der Zorn Gottes Capote Grizzly man Notes on a scandal
This is a file, containing some film titles. In the code example, we are going to read it.
#!/usr/bin/python3 # try_except_finally.py f = None try: f = open('films', 'r') for i in f: print(i, end="") except IOError: print("Error reading file") finally: if f: f.close()
We try to read a films file. If no exception occurs, we print the contents
of the file to the console. There might be an exception. For example, if we
provided an incorrect file name. In such a case a
exception is raised. The
except keyword catches the
exception and executes its block of code. The
finally keyword is
always executed in the end. We use it to clean up our resources.
In the next example, we show how to create a user defined exception
#!/usr/bin/python3 # raise_kwd.py class YesNoException(Exception): def __init__(self): print('This is not a yes or no answer') answer = 'y' if (answer != 'yes' and answer != 'no'): raise YesNoException else: print('Correct value')
In the example, we expect only yes/no values. For other possibilities, we raise an exception.
$ ./raise_kwd.py This is not a yes or no answer Traceback (most recent call last): File "./raise_kwd.py", line 15, in <module> raise YesNoException __main__.YesNoException
del keyword deletes objects.
#!/usr/bin/python # del_kwd.py a = [1, 2, 3, 4] print(a) del a[:2] print(a)
In our example, we have a list of four integer numbers. We delete the first numbers from the list. The outcome is printed to the console.
$ ./del_kwd.py [1, 2, 3, 4] [3, 4]
This is the output of the example.
pass keyword does nothing. It is a very handy
keyword in some situations.
def function(): pass
We have a function. This function is not implemented yet. (It will be later.) The body of the function must not be empty. So we can use a pass keyword here, instead of printing something like "function not implemented yet", or similar.
assert keyword is used for debugging purposes.
We can use it for testing conditions that are obvious to us. For example,
we have a program that calculates salaries. We know that the salary cannot
be less than zero. So we might put such an assertion to the code. If the
assertion fails, the interpreter complains.
#!/usr/bin/python3 # assert_kwd.py salary = 3500 salary -= 3560 # a mistake was done assert salary > 0
During the execution of the program a mistake was done. The salary becomes a negative number.
$ ./assert_kwd.py Traceback (most recent call last): File "./assert_kwd.py", line 8, in <module> assert salary > 0 AssertionError
The execution of the script fails with the
class keyword is used to create new user defined objects.
#!/usr/bin/python3 # class_kwd.py class Square: def __init__(self, x): self.a = x def area(self): print(self.a * self.a) sq = Square(12) sq.area()
In the code example, we create a new
Square class. Then we instantiate
the class and create an object. We compute the area of the square object.
exec keyword executes Python code dynamically.
#!/usr/bin/python3 # exec_kwd.py exec("for i in [1, 2, 3, 4, 5]: print(i, end=' ')")
We print five numbers from a list using a for loop; all within
$ ./exec_kwd.py 1 2 3 4 5
This is the example output.
Next, we mention the
in keyword. The keyword tests
whether a value is present in a sequence.
#!/usr/bin/python3 # in_kwd.py print(4 in (2, 3, 5, 6)) for i in range(25): print(i, end=" ") print()
In this example, the in keyword tests if the number four is in the tuple.
The second usage is traversing a tuple in a for loop. The built-in function
range() returns integers 0 .. 24.
$ ./in_kwd.py False 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
yield keyword is used with generators.
#!/usr/bin/python3 # yield_kwd.py def gen(): x = 11 yield x it = gen() print(it.__next__())
yield keyword exits the generator and returns a value.
$ ./yield_kwd.py 11
In this part of the Python tutorial, we have covered Python keywords.