Python – Lecture 3
S.Venkatesan
Network Security and Cryptography Lab
Department of Information Technology
Indian Institute of Information Technology, Allahabad
venkat@iiita.ac.in
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Acknowledgement: The example scripts and some figures are copied from various sources. Thanks to
all authors and sources which made those contents public and usable for educational purpose
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Frozenset
• The frozenset() is an inbuilt function is Python which takes an
iterable object as input and makes them immutable. Simply it
freezes the iterable objects and makes them unchangeable.
Syntax
frozenset(iterable_object_name)
Example
Student = {"name”, "Ankit", "age”, "Allahabad"}
key = frozenset(Student)
print(key)
Output
({‘name’, ’Ankit,’ ’age’, ’Allahabad’})
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Dictionary
• A dictionary is a collection which is unordered,
changeable and indexed. In Python dictionaries are
written with curly brackets, and they have keys and
values.
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = thisdict["model"]
x = thisdict.get("model”)
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Dictionary-Examples
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict["color"] = "red"
print(thisdict)
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thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.pop("model")
print(thisdict)
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Dictionary-Examples
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
del thisdict
print(thisdict)
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.clear()
print(thisdict)
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = thisdict.copy()
print(mydict)
You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will
only be a reference to dict1, and changes made in dict1 will automatically also
be made in dict2.
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Functions
• A function is a block of code which only runs when it is called.
• You can pass data, known as parameters, into a function and
return data as a result.
• Syntax
def function-name(arguments):
statement 1
statement 2
Optional
.
statement n
return variable
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Example
def my_function(fname, lname):
print(fname + " " + lname)
my_function("Emil", "Refsnes")
def my_function(child3, child2, child1):
print("The youngest child is " + child3)
my_function(child1 = "Emil", child2 = "Tobias",
child3 = "Linus")
def my_function(country = "Norway"):
print("I am from " + country)
my_function("Sweden")
my_function("India")
my_function()
my_function("Brazil")
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Arguments ( Keyword Arguments, **kwargs &
Arbitrary Arguments, *args) & Return
def my_function(*kids):
print("The youngest child is " + kids[2])
my_function("Emil", "Tobias", "Linus")
def my_function(**kid):
print("His last name is " + kid["lname"])
my_function(fname = "Tobias", lname = "Refsnes")
def my_function(x):
return 5 * x
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Recursion
def tri_recursion(k):
if(k > 0):
result = k + tri_recursion(k - 1)
print(result)
else:
result = 0
return result
print("\n\nRecursion Example Results")
tri_recursion(6)
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import
In Python, a module is a self-contained file with Python statements and definitions. For example, file.py, can be
considered a module named file.
hello.py
def my_function():
print("Hello World")
import hello
hello.my_function()
from hello import my_function
my_function()
hello.py
def my_function():
print("Hello World")
name = "Nicholas“
import hello
hello.my_function()
print(hello.name)
Question: module vs package
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Lambda function
•
•
A lambda function is a small anonymous function.
A lambda function can take any number of arguments, but can only have one
expression.
Syntax:
lambda arguments : expression
x = lambda a : a + 10
print(x(5))
x = lambda a, b : a * b
print(x(5, 6))
•
Advantage: A small "disposable" one-time-use utility code, ordinary functions
don't work so well. They can actually makes the code significantly less readable.
This is where lambda functions come in. They allow one to inject that disposable
utility code straight into the point of the call, where it is necessary.
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Map function
•
This function returns a map object(which is an iterator) of the results after applying the given
function to each item of a given iterable (list, tuple etc.)
•
Syntax
map(function, iterable)
•
PARAMETERS
–
–
•
Function
Numbers (iterable)
Return Value
–
List
def addition(n):
return n + n
# We double all numbers using map()
numbers = (1, 2, 3, 4)
result = map(addition, numbers)
print(list(result))
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Filter
• It requires the function to return boolean values (true or false) and then
passes each element in the iterable through the function, "filtering" away
those that are false.
• syntax:
filter(func, iterable)
• func argument is required to return a boolean type.
# Python 3
scores = [66, 90, 68, 59, 76, 60, 88, 74, 81, 65]
def is_A_student(score):
return score > 75
over_75 = list(filter(is_A_student, scores))
print(over_75)
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Reduce function
• It is defined in the functools module. Like the map and
filter functions, the reduce() function receives two
arguments, a function and an iterable. However, it
doesn't return another iterable, instead it returns a
single value.
import functools
def mult(x,y):
print("x=",x," y=",y)
return x*y
fact=functools.reduce(mult, range(1, 4))
print ('Factorial of 3: ', fact)
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Python main function
• Some programming languages have a special function
called main() which is the execution point for a program
file. Python interpreter, however, runs each line serially
from the top of the file and has no explicit main() function.
• A special variable called __name__ provides the
functionality of the main function. As it is an in-built
variable in python language.
def func1():
print 'The value of __name__ is ' + __name__
if __name__ == '__main__':
func1()
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Underscore
• Leading double underscore tell python
interpreter to rewrite name in order to avoid
conflict in subclass.
• Interpreter changes variable name with class
extension and that feature known as the
Mangling.
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Web References
https://www.geeksforgeeks.org/frozenset-in-python/
https://www.w3schools.com/python/python_dictionaries.asp
https://www.w3schools.com/python/python_functions.asp
https://www.w3schools.com/python/python_lambda.asp
https://www.geeksforgeeks.org/python-map-function/
https://stackoverflow.com/questions/18168022/what-is-the-advantageof-lambda-expressions
• https://www.programiz.com/python-programming/methods/built-in/map
• https://www.tutorialsteacher.com/python/python-reduce-function
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All the above are accessed between – 17 to 19 November 2020
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