Intro to Python

Introduction

Text

Why Python?

Easy to Learn

  • Concise

  • Well-Documented

Well-Documented

796,949 Questions

Concise

print('hello, world')
#include<iostream>
 
using namespace std;
 
int main()
{
	cout << "Hello World\n";
	return 0;
}

c++

python

Open Sourced

  • Free to Use

  • Numerous Libraries

Libraries

numpy

scipy

matplotlib

openCV

django

pygame

scikit-learn

pandas

Syntax I

Function Basics


# Comments are preceded by the '#' symbol
        
    

# Comments are preceded by the '#' symbol

"""
Comments blocks that span multiple lines
can be denoted with a pair of three quotes
"""

# Comments are preceded by the '#' symbol

"""
Comments blocks that span multiple lines
can be denoted with a pair of three quotes
"""

'''
Either double or single quotes can be used
'''
        
    

# Functions are defined like so


# Functions are defined like so
# def FUNCTION_NAME( PARAMETERS ):




# Functions are defined like so
# def FUNCTION_NAME( PARAMETERS ):


# In good practice, a comment or comment block 
# precedes a function, describing its purpose
def myFirstFunction( a, b ):  

 

# Functions are defined like so
# def FUNCTION_NAME( PARAMETERS ):


# In good practice, a comment or comment block 
# precedes a function, describing its purpose
def myFirstFunction( a, b ):  

    # The code body that follows must be indented
    a = a + 2

    

# Functions are defined like so
# def FUNCTION_NAME( PARAMETERS ):


# In good practice, a comment or comment block 
# precedes a function, describing its purpose
def myFirstFunction( a, b ):  

    # The code body that follows must be indented
    a = a + 2

    # Since variables are implicitly declared, 
    # a type, like 'int', is not needed to instantiate 'c' 
    c = a + b

    

# Functions are defined like so
# def FUNCTION_NAME( PARAMETERS ):


# In good practice, a comment or comment block 
# precedes a function, describing its purpose
def myFirstFunction( a, b ):  

    # The code body that follows must be indented
    a = a + 2

    # Since variables are implicitly declared, 
    # a type, like 'int', is not needed to instantiate 'c' 
    c = a + b

    # In Python any layer of logical abstraction 
    # (e.g. class, function, conditional) is denoted 
    # by a colon and followed by further indentation
    if c > 10:
        c = c * 2
        print(c)

Function Basics

Let's run our script and

see what happens...


def myFirstFunction( ):  
    print('Hello, World!')

Input

Output

What happened?

We need to call our function!


def myFirstFunction( ):  
    print('Hello, World!')


myFirstFunction()

Hello, World!

Success!

Function Basics

Like that of most languages, python's functions return a value


def mySecondFunction( ):
    return 4

a = mySecondFunction()
print(a)


4

Input

Output

We can return other types too

By default, this value is None,

but we can specify our own  value


def myThirdFunction( ):
    return 'python is great!'

a = myThirdFunction()
print(a)


python is great!

Function Basics

In a script we typically define a main function we invoke to execute our other functions







def func1( ):
    return 5

def func2( a, b ):  
    print(a + b)




11

Input

Output


def main( ):
    foo = func1()    # foo := 5
    bar = 6
    func2(foo, bar)    

def func1( ):
    return 5

def func2( a, b ):  
    print(a + b)

main()

Print Statements

Python makes print statements easy


>>> print(10)



>>> print(10)
10 


>>> print(10)
10 

>>> foo = 10
>>> print(foo)


>>> print(10)
10 

>>> foo = 10
>>> print(foo)
10

Simple, right?


>>> print('cat')

>>> print('cat')
cat


>>> print('cat')
cat

>>> bar = 'cat'
>>> print(cat)


>>> print('cat')
cat

>>> bar = 'cat'
>>> print(cat)
cat

Print Statements

By default, print() will output a new line at the end


def main():
    foo = 'dog'
    bar = 'turtle'
    print(foo)
    print(bar)
    print(bar)

main()

dog
turtle
turtle

Input

Output

We can suppress this using commas


def main():
    foo = 'dog'
    bar = 'turtle'
    print(foo),
    print(bar),
    print(bar),

main()

dog turtle turtle

Strings

String concatenation is similar to that of other languages


>>> foo = 'cat'
>>> bar = 'dog'
>>> foo + bar


>>> foo = 'cat'
>>> bar = 'dog'
>>> foo + bar
'catdog'

String formatting is in C-style


>>> foo = 'cat'
>>> s1 = 'I have a %s at home.' % (foo)
>>> s1


>>> foo = 'cat'
>>> s1 = 'I have a %s at home.' % (foo)
>>> s1
'I have a cat at home.'

>>> foo = 'cat'
>>> s1 = 'I have a %s at home.' % (foo)
>>> s1
'I have a cat at home.'

>>> bar = 'dog'
>>> s2 = 'I have a %s and a %s at home.' % (foo, bar)
>>> s2

>>> foo = 'cat'
>>> s1 = 'I have a %s at home.' % (foo)
>>> s1
'I have a cat at home.'

>>> bar = 'dog'
>>> s2 = 'I have a %s and a %s at home.' % (foo, bar)
>>> s2
'I have a cat and a dog at home.'

We use %s since foo and bar are both strings, but we can substitute other types too


>>> foo = 20
>>> s1 = 'I am $d years old.' % (foo)
>>> s1



>>> foo = 20
>>> s1 = 'I am $d years old.' % (foo)
>>> s1
'I am 20 years old.'




>>> foo = 20
>>> s1 = 'I am $d years old.' % (foo)
>>> s1
'I am 20 years old.'

>>> bar = 'dogs'
>>> s2 = 'I have %d %s at home.' % (foo, bar)
>>> s2

>>> foo = 20
>>> s1 = 'I am $d years old.' % (foo)
>>> s1
'I am 20 years old.'

>>> bar = 'dogs'
>>> s2 = 'I have %d %s at home.' % (foo, bar)
>>> s2
'I have 20 dogs at home.'

There are several other formatting operators, but for this class, all we'll need to know are %s, %d and %f for

string, integer and float, respectively

A more comprehensive guide can be found here

Arithmetic

Python uses standard mathematical operators

+     -     *     /

Recognize them?

Sure hope so!

Additionally, Python uses %

for modulus division

and **

for exponents

Let's look at some examples of usage


>>> 3 % 2



>>> 3 % 2
1

>>> 3 % 2
1

>>> 5 % 5


>>> 3 % 2
1

>>> 5 % 5
0

>>> 3 % 2
1

>>> 5 % 5
0

>>> 17 % 3

>>> 3 % 2
1

>>> 5 % 5
0

>>> 17 % 3
2

>>> 3 % 2
1

>>> 5 % 5
0

>>> 17 % 3
2

>>> 4 % 6

>>> 3 % 2
1

>>> 5 % 5
0

>>> 17 % 3
2

>>> 4 % 6
4

>>> 3 ** 2

>>> 3 ** 2
9

>>> 3 ** 2
9

>>> 4 ** 3


>>> 3 ** 2
9

>>> 4 ** 3
64


>>> 3 ** 2
9

>>> 4 ** 3
64

>>> 2 ** 1.4

>>> 3 ** 2
9

>>> 4 ** 3
64

>>> 2 ** 1.4
2.6390158215457884

It's okay if you didn't get

that last one

Syntax II

Conditionals


# A basic conditional is structured like so..

Like c++, python has

conditionals and loops


# A basic conditional is structured like so..

"""
if CONDITION:
    STATEMENT

"""

# A basic conditional is structured like so..

"""
if CONDITION:
    STATEMENT

"""

# Example 1
# This statement will execute since True is always True
if True: 
    print('Hello, World!')

# A basic conditional is structured like so..

"""
if CONDITION:
    STATEMENT

"""

# Example 1
# This statement will execute since True is always True
if True:
    print('Hello, World!')

# Example 2
# This statement will not execute since (5 < 4) is False
if 5 < 4:
    print('Hello, World!')

# We can also append if-else and else conditions like so..

"""












"""

# We can also append if-else and else conditions like so..

"""
if CONDITION_1:
    STATMENT_1










"""

# We can also append if-else and else conditions like so..

"""
if CONDITION_1:
    STATMENT_1

elif CONDITION_2:
    STATEMENT_2







"""

# We can also append if-else and else conditions like so..

"""
if CONDITION_1:
    STATMENT_1

elif CONDITION_2:
    STATEMENT_2

elif CONDITION_3:
    STATEMENT_3




"""

# We can also append if-else and else conditions like so..

"""
if CONDITION_1:
    STATMENT_1

elif CONDITION_2:
    STATEMENT_2

elif CONDITION_3:
    STATEMENT_3

else:
    STATEMENT_4

"""

Conditionals


Statement 3 Executed

a = 2
b = 4
input = 5

Input

Output

Examples

Conditions 1 and 2 are False so statement 3 executes


Statement 1 Executed

Statement 1 is the first True statement so it, alone, executes


a = 2
b = 4
input = 5

if input < a:
    print('Statement 1 Executed')
elif input < b:
    print('Statement 2 Executed')
else:
    print('Statement 3 Executed')

a = 6
b = 99
input = 5

if input < a:
    print('Statement 1 Executed')
elif input < b:
    print('Statement 2 Executed')
else:
    print('Statement 3 Executed')

Guess the output!

Conditionals

Same old comparison operators

==    !=    <    <=    >    >= 

And some alternatives too!

Alternative

is

not

<>

and

or

Original

==

!

!=

&&

||


if True || False:
    print('Lets go Terps!')

if !(True && False):
    print('I am a Flat-Earther')

Original

Alternatively..


if True or False:
    print('I like toitles')

if not (True and False):
    print('I am ignorance')

if a != b:
    print('I am a business major!')

How about this one?

These statements are logically equivalent


# first possibility
if a is not b:
    print('I am a business major!')


# first possibility
if a is not b:
    print('I am a business major!')

# second possibility
if a <> b:
    print('I am a business major!')

There's a slight discrepancy between the two

We'll go over that in a bit!

Try and guess the alternatives!

Conditionals


John is a winner.

a = 10
b = 7

john = 'winner' if a > b else 'loser'
print('Josh is a %s.' % john)

Input

Output

Conditionals can also be used for assignment

If 'a' was less than 'b' we would have assigned the latter to 'john'

Good thing 'a' was greater!


# example 1
if a > b:
    john = 'winner'
else:
    john = 'loser'

# example 2
john = 'winner' if a > b else 'loser'

The two examples to the left are logically equivalent

In-line boolean assignments can be used to 'clean up' our code and make it more readable

Loops

While-loops have a similar structure to if-statements


# Structure a while-loop like so..



# Structure a while-loop like so..



# Structure a while-loop like so..

"""
while CONDITION:
    STATEMENT

"""


# Structure a while-loop like so..

"""
while CONDITION:
    STATEMENT

"""

# Example 1


# Structure a while-loop like so..

"""
while CONDITION:
    STATEMENT

"""

# Example 1
# This loop will execute infinitely since True is always True
while True:
    print('Hello, World!')


# Structure a while-loop like so..

"""
while CONDITION:
    STATEMENT

"""

# Example 1
# This loop will execute infinitely since True is always True
while True:
    print('Hello, World!')

# Example 2

# Structure a while-loop like so..

"""
while CONDITION:
    STATEMENT

"""

# Example 1
# This loop will execute infinitely since True is always True
while True:
    print('Hello, World!')

# Example 2
# This loop will execute twice
count = 0
while count < 2:
    count += 1
# We can also use break statements to exit a loop prematurely


# We can also use break statements to exit a loop prematurely

# Example 3
# We can also use break statements to exit a loop prematurely

# Example 3
# This loop will terminate before printing anything
while True:
    break
    print('Hello, World!')


# We can also use break statements to exit a loop prematurely

# Example 3
# This loop will terminate before printing anything
while True:
    break
    print('Hello, World!')

# Example 4
# We can also use break statements to exit a loop prematurely

# Example 3
# This loop will terminate before printing anything
while True:
    break
    print('Hello, World!')

# Example 4
# How about this one?
count = 0
while count < 3:
    print('Hello, World!')
    if count == 1:
        break
    count += 1
# We can also use break statements to exit a loop prematurely

# Example 3
# This loop will terminate before printing anything
while True:
    break
    print('Hello, World!')

# Example 4
# How about this one?
count = 0
while count < 3:
    print('Hello, World!')
    if count == 1:
        break
    count += 1
# This one will print 'Hello, World!' twice
# We can also use break statements to exit a loop prematurely

# Example 3
# This loop will terminate before printing anything
while True:
    break
    print('Hello, World!')

# Example 4
# How about this one?
count = 0
while count < 3:
    print('Hello, World!')
    if count == 1:
        break
    count += 1
# This one will print 'Hello, World!' twice
# Can you see why?

Loops

For-loops are another type of loop

Typically, they are preferred when the number of iterations is known beforehand


# Structure a for-loop like so..

# Structure a for-loop like so..

"""
for VARIABLE in SPAN:
    STATEMENT

"""

# Structure a for-loop like so..

"""
for VARIABLE in SPAN:
    STATEMENT

"""

# The span can be a range of numbers
for i in range(4):
    print(i)


# Structure a for-loop like so..

"""
for VARIABLE in SPAN:
    STATEMENT

"""

# The span can be a range of numbers
for i in range(4):
    print(i)

# or a predefined list
arr = [1, 4, 6]
for elem in arr:
    print(elem)

# Structure a for-loop like so..

"""
for VARIABLE in SPAN:
    STATEMENT

"""

# The span can be a range of numbers
for i in range(4):
    print(i)

# or a predefined list
arr = [1, 4, 6]
for elem in arr:
    print(elem)

# Don't worry about outputs. We'll cover that in a minute
# The span doesn't have to be comprised of numbers

# The span doesn't have to be comprised of numbers

# We can even loop over a list of strings
students = ['Alice', 'Bob', 'Charlie']
for name in students:
    print(name)

# The span doesn't have to be comprised of numbers

# We can even loop over a list of strings
students = ['Alice', 'Bob', 'Charlie']
for name in students:
    print(name)

# or a list of booleans
bools = [True, False, True, True]
for val in bools:
    if val:
        ...

# The span doesn't have to be comprised of numbers

# We can even loop over a list of strings
students = ['Alice', 'Bob', 'Charlie']
for name in students:
    print(name)

# or a list of booleans
bools = [True, False, True, True]
for val in bools:
    if val:
        ...

# This allows to effectively represent complex logic 
count = 0
for p in points:
    if p > 100:
        count += 1
print('%d players scored over 100 points!' % count)

Loops

Examples


students = ['Alice', 'Bob', 'Charlie']
for name in students:
    print(name)

Input

Output

The for-loop iterates through our students list and stores the value of each element to name


Alice
Bob
Charlie

On each successive pass it prints the current value of name


bools = [True, False, True, True]
for val in bools:
    if val:
        print(val)



True
True
True

Here, only True elements activate the if-statement 


points = [156, 97, 103, 245, 53]
count = 0
for p in points:
    if p > 100:
        count += 1
print(('%d players scored over 100' 
    ' points!') % count)

3 players scored over 100 points!

We start with count := 0 and increment by 1 each time we see an element of points greater than 100 

Note that the print statement lies outside the for-loop

Modules

Module (n) /ˈmäjool/

each of a set of standardized parts or independent units that can be used to construct a more complex structure

Importing

We need to import modules before using them

# It's best practice to import them at the top of
# your script
import math
# It's best practice to import them at the top of
# your script
import math


# Once declared, module functions can be used like so
# MODULE.FUNCTION(PARAMETERS)
>>> import math
>>> a = math.floor(4.5)
>>> a
4.0

Easy, right?

# Multiple modules can be called in-line
# Delimit them with commas
import math, random, csv


# Multiple modules can be called in-line
# Delimit them with commas
import math, random, csv


# Module names can also be aliased, like so,
# for improved readability
>>> import math as m
>>> a = m.floor(4.5)
>>> a 
4.0

All the modules of Python's Standard Library are readily available with your Python installation

You can find a list of those modules here

Modules that are not a part of the Standard Library can be installed through the terminal using pip

References

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