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

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)

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)

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

not

and

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

Intro to Python

Introduction

Why Python?

Easy to Learn

Concise

Well-Documented

Well-Documented

Concise

c++

python

Open Sourced

Free to Use

Numerous Libraries

Libraries

Syntax I

Function Basics

Function Basics

Function Basics

Function Basics

Print Statements

Print Statements

Strings

Arithmetic

Syntax II

Conditionals

Conditionals

Input

Output

Conditionals

Original

Alternatively..

Conditionals

Input

Output

Loops

Loops

Loops

Modules

Module (n) /ˈmäjool/

Importing

References

Lec 1 - Intro to Python

More from Brian J Lee