Intro to Python

Python Crash Course

Objectives

Describe the history of the Python language
Identify fundamentals and concepts of the Python language
Utilize different primitive types, control structures, and methods in Python
Use the python3 interactive shell
Write a program that responds to user input

Python History

Benevolent Dictator For Life

Guido van Rossum (Dutch) built Python as a hobby project on his winter break

Goals

An easy and intuitive language just as powerful as major competitors
Open source, so anyone can contribute to its development
Code that is as understandable as plain English
Suitability for everyday tasks, allowing for short development times

Releases

Python 1.0 realeased 1991
Python 2.0 (2000) - started to gather a development community
Python 3.0 (2008) - unified the language more at the cost of making some old 2.0 code incompatible

Advantages

high-level language (closer to english than machine code)
interpreted (as opposed to compiled)programming language (compiled languages have to be run through a compiler that changes
source code into compiled code before the program can be run, whereas interpreted
languages are run on the fly
general purpose scripting language that runs outside a browser
free and open source (community development through Python Enhancement Proposals)
comes with a huge library of builtin functions
The documentation is fantastic

Python Features and Syntax!

Here's the long list of how to get Python to do the things you already know how to do in JavaScript. Let's dive right into it.

Comments

In Python, multiline comments exist, but we generally use line comments with hashtags, for readability.

# here is a line comment

# here is a block
# of comments
# for you to read

For multiline comments in Python, you can use """ blocks. However observe proper indentation or you will get a SyntaxError.

Variables

Local variables start with a lowercase letter. No var necessary. Most Python programs choose to insert underscores between words instead of writing names in camelCase (this is considered more readable by the PEP maintainers).

my_variable = 5
print(my_variable)

Data Types

Nothingness

Just as Javascript uses undefined or null, Python uses None.

None is a unique datatype (NoneType) and is not equivalent to False, empty string, or 0. However, None does not evaluate to True.

my_bank_account = None

if my_bank_account:
   print("This is true!")
else:
   print("This is not true...")

# will print "This is not true..."

Booleans

A binary representation: either True or False. Capitalization matters here. you must write "True" and Python will not recognize "true."

is_operating = True
is_broken = False

Numbers

There are several datatypes used to represent numbers:

int: 23
float: 0.8947368421052632
complex: 2+3j

Python does decimal division normally with / but you can force it to do integer division with //:

Complex numbers can be added to complex numbers:

complex = 1+2j + 5 + 3j # evaluates to (6+5j)

Python is great at dealing with very large integers. Use ** to calculate exponents. JavaScript will only approximate large numbers with Scientific Notation:

// JavaScript
Math.pow(2, 999)
5.357543035931337e+300

# Python
2 ** 999
5357543035931336604742125245300009052807024058527668037218751941851755255624680612465991894078479290637973364587765734125935726428461570217992288787349287401967283887412115492710537302531185570938977091076523237491790970633699383779582771973038531457285598238843271083830214915826312193418602834034688

Strings

You can declare strings in Python just like in JavaScript:

card1 = "ace of spades"
card2 = "king of hearts"

String Methods

Python has some familiar String methods like split. Other String methods are under different names:

"ace of spades".split(" ")
['ace', 'of', 'spades']

"abcde".split("")

"qqxzzz".index("x")
2

"boo".upper()
"BOO"

"WHY???".lower()
"why???"

"then I went to the store I like".replace("I", "you")
'then you went to the store you like'

You can use the special in keyword to quickly find out if one string appears in another.

"eggs" in "green eggs and ham"
True

Use the len() operator on a string to find it's length.

len("Hawaii")
6

Ranges

Python has a very cool syntax to select things inside a range. You can select a letter in a string a specific index. You can select letters between a start and and index. You can select letters counting backwards from the end using negative numbers. You can select every N letters by specifying a step size.

str[index] choose one letter at index
str[-index] choose letter at index counting backwards from the end.
str[start:end] get a range of letters from a start to end.
str[start:end:step] get a range of letters taking step sized steps between.
str[:end] omit the start index and grab letters from zero up to end.
str[start:] omit the end index and grab letters from start up to the end of the string.
str[::-1] reverses a string by going backwards with a step of -1 from start to end.

# get the last letter
"my code rulez"[-1]
"z"

"my code rulez"[3:7]
"code"

"my code rulez"[:2]
"my"

"my code rulez"[3:]
"code rulez"

"my code rulez"[::-1]
"zelur edoc ym"

# take every other letter
"peiege eleaeteiene"[::2]
'pig latin'

Arithmetic Operators

Python uses common operators for math.

+
-
/    # decimal division       4 /  3 == 1.333333
//   # force integer division 4 // 3 == 1
*
**   # exponent  2 ** 3 == 27
%    # modulo    17 % 5 == 2

+=
-=
*=
/=
**=

Logical Operators

Python uses == for equality. It uses != for "not equal to." It uses the literal words not, or and and instead of !, || and && like JavaScript does.

==
!=
not
or
and

Lists

Arrays in Python are called Lists.

Python lists can be indexed just like strings. Also, use the len() operator to find the length of a list. Python lists don't have a .length property on them.

Python lists can be indexed just like Python strings. You can use negative indexes and select a range of values in the middle of the list.

several ways to create an array

arr = [1,2,3]
[1,2,3]

len(arr)
3

five_zeroes = [0] * 5
[0, 0, 0, 0, 0]

len(five_zeroes)
5

five_trues = [True] * 5
[True, True, True, True, True]

last_item = ["Washington", "Oregon", "California"][-1]
"California"

# generate a range from 1 to 10 and create list out of it.
one_through_ten = list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

List Methods

a = [1, 23, 12, 99, 82]
a.sort()
[1, 12, 23, 82, 99]

# reverse the list
a = a[::-1]
[99, 82, 23, 12, 1]

a.append(42)
[99, 82, 23, 12, 1, 42]

result = a.pop() # the last value is stored in the variable

# notice: the value is actually removed from the array
print(a)
[82, 23, 12, 1, 42]

# insert a value at a specific index
a.insert(0, 123)
[123, 82, 23, 12, 1, 42]

a.insert(2, 66)
[123, 82, 66, 23, 12, 1, 42]


if 42 in a:
  print("yup! it's in there.")

Fire up the interactive Python console on your terminal with python3 or ipython3 and pass list to the help function to read documentation about all of the methods on list.

help(list)

Dictionaries

Python stores key-value pairs (like objects in JavaScript) in Dictionaries. The syntax is almost exactly like objects in JavaScript.

Anything can be used as a key. Strings are commonly used as keys. If the key is a string then you must surround the key in quotes when the dictionary is defined.

cat = {
  "name": 'Hamlet',
  "breed": 'American Shorthair',
  "fav_food": 'Prosciutto'
}

cat["name"]
'Hamlet'

cat["fav_food"]
'Prosciutto'

Type Conversion

You can turn one Python data type into another by passing it to type functions:

int("42")
42

str(42)
"42"

float(42)
42.0

bool(42)
True

bool(0)
False

bool("foo")
True

bool("")
False

String Interpolation

Python has super convenient ways to format strings on the fly. You'll never have to bother with crazy string concatenation again. These are like small rendered templates.

Define any string with the letter f in front of the quotes and Python will treat it as a "formatted" string. Formatted strings use curly braces {} to identify where to evaluate values inside a string. The value inside the curly braces can be variable names, or expressions.

The curly braces and the format function can refer to names for where values appear in the template, and what values are named.

state = "Washington State"
year = 1889
n = 42
my_message = f"{state} was the {n}th state to join the union in {year}."

Let's see format strings calling a function too!

def st_nd_rd_th(n):
  # add one to 13 because range is exclusive at the end.
  if n in range(11, 13 + 1):
    return "th"
  if n % 10 == 1:
    return "st"
  elif n % 10 == 2:
    return "nd"
  elif n % 10 == 3:
    return "rd"
  else:
    return "th"

state = "Washington State"
year = 1889
n = 42
my_message = f"{state} was the {n}{st_nd_rd_th(n)} state to join the union in {year}."
print(my_message)

F-strings are awesome. There's also a format() function on all strings. You can specify empty curly braces and pass values as parameters and the parameter values will fall in line in the order the curly braces appear.

template = "My name is {} and I like {}"
template.format("Steve", "Cheese")
'My name is Steve and I like Cheese'

The format() function an also name the curly braces and accept named parameters:

template = "My name is {name} and I like {food}"
template.format(food="Cheese", name="Steve")
'My name is Steve and I like Cheese'

Control flow

Conditionals

vip = True
food_place = "busy"
if (food_place == "full" and not vip):
  print("Sorry, we have no room tonight.")
elif (food_place == "busy" and not vip)
  print("Please wait 10 minutes for a table.")
else:
  print("Welcome! Come sit down right away.")

Loops

n = 0
while n < 10:
  n++

for i in range(0, 10):
  if i % 2 == 0:
    print("{} is even".format(i))
  if i % 2 == 1:
    print("{} is odd".format(i))

Iterating through Arrays

Python for loops always act on iterators. This means they always automatically pull things out of a sequence and loop through those things one by one. Python never uses a for loop where it increments a value like i manually.

If you want to have fine-grain control over what's happening to i, and you don't want to just step through everything in a sequence one by one, then you're probably better off using a while loop.

foods = ["carrots", "kale", "beets"]
for food in foods:
  print("I like", food)

If you want access to the current index of each item then you need to pass your list through a function called enumerate. Enumerate takes items out of an iterator one by one and returns a tuple of the index of the item and the item like (index, item).

print("My favorite foods:")
foods = ["carrots", "kale", "beets"]
for i, food in enumerate(foods):
  print("{}. {}".format(i, food))

i is still zero-indexed:

My favorite foods:
0. carrots
1. kale
2. beets

Iterating through Dictionaries

For loops will automatically iterate over the keys in a dictionary:

car = {"wheels": 4, "doors": 2, "seats": 5}
for key in car:
  print("my car has {num} {thing}".format(thing=key, num=car[key]))

# Will print out:
# my car has 4 wheels
# my car has 2 doors
# my car has 5 seats

Functions

In Javascript

anonymous: function (param1, [..param2, [...]]){...},
named: function Name(param1, [..param2, [...]]){...}
uses lexical scope
used as values (functional programming)
require explicit return
all params are optional
no default values for optional params
functions can be used before they're declared

In Python

uses def foo(param1, param2, city="Seattle"):
may provide default values for optional parameters
may use one return statement to return more than one variable
functions must be declared before they're used

Examples

def say_hello():
  print("Hello, World!")

say_hello()

Parameters (Arguments)

def say_hello(friend="Tim"):
  print("Hello, {}!".format(friend))

def move(name, city="Seattle", state="Washington"):
  msg = "{} is moving to {}, {}"
  msg = msg.format(name, city, state)
  print(msg)

move("Charlie")
"Charlie is moving to Seattle, Washington"

# values passed to functions will fall in parameter order.
# parameter default values are used if nothing is passed in at that spot.
move("Charlie", "Tacoma")
"Charlie is moving to Tacoma, Washington"

move("Charlie", "Boise", "Idaho")
"Charlie is moving to Boise, Idaho"

# use names to specify exactly what parameters you're giving values to.
move("Charlie", state="Oregon")
"Charlie is moving to Seattle, Oregon"

move("Charlie", city="Boise", state="Idaho")
"Charlie is moving to Boise, Idaho"

# if you're using a named parameter, then you can mix up the order!
move("Charlie", state="Oregon", city="Portland)
"Charlie is moving to Portland, Oregon"


# Any value passed to the function will be used.
# If no value is passed for a parameter Python uses the default value
# defined in the function.
say_hello("Ronald")
say_hello("Tim")
say_hello()

Return Values

def add(num1, num2):
  return num1 + num2

total = add(2, 3)
print("2 + 3 =", total)

Tuples and Multiple Return Values

Python has a datatype called a tuple which groups "multuple" things together. A tuple is a collection of multiple values wrapped in parentheses. Tuples are not lists. List support more operations than tuples. Tuples are immutable. Once one is made it never changes. You can't add or remove things from a tuple.

Python will automatically unpack tuples into variables if the number of variables on the left side of an equals sign is equal to the number of variables on the right hand side.

tuple = ("Fireman", "Fire Department")
job_title, office = tuple

Python uses tuples to return multiple values in one return statement. Here's our own function called divide_mod() that allows us to calculate the division of two numbers as well as their modulus all at once!

def divide_mod(numerator, denominator):
  division = numerator // denominator
  mod = numerator % denominator
  return (division, mod)

students = 11
group_size = 4
num_groups, left_over = divide_mod(students, group_size)
msg1 = "{n} students can be divided into {m} groups of {size}"
msg2 = "with {j} students left over."

msg1 = msg1.format(n=students, m=num_groups, size=group_size)
msg2 = msg2.format(j=left_over)

print(msg1)
print(msg2)

11 students can be divided into 2 groups of 4
with 3 students left over.

Input / Output

You've already seen how print will output information to the screen. What if e want to accept user input? Let's try input(). Input takes a parameter where we can define a message to be printed as a prompt for the input:

def gather_input():
  you = input("Enter your name: ")
  friend = input("Enter a friend's name: ")
  print("Hello, {}. {} says hi.".format(you, friend))

gather_input()

# Outputs
# Enter your name: Tim
# Enter a friend's name: Bob
# Hello, Bob. Tim says hi.

Notice: input() always returns a string value. If you want someone to enter a number then you should pass the value input returns through something like int() or float().

age = input("how old are you?")
age = int(age)

age = int(input("how old are you?"))

cost = input("how much was that candy bar?")
cost = float(cost)

cost = float(input("how much was that candy bar?"))

There's nothing special about the int() or float() functions. They try to convert whatever value their given to an int or a float.

file_data = "2007,2017,Seattle"
moved_year, current_year, city = tupple(file_data.split(","))

# Convert years from strings to ints
moved_year = int(moved_year)
current_year = int(current_year)

diff = current_year - moved_year

print("Someone moved to {city} in {moved_year} and has lived there for {diff} years."

Now You're Hacking!

That's a quick introduction to only some of the things Python has to offer. You now have enough knowledge about Python's basic data types, functions, lists, dictionaries, control flow statements to try writing some programs of our own.

Try to write a program that prompts the user for the current temperature, and asks them whether they've entered a Celsius or Fahrenheit temperature. Have the program convert their temperature from the units they provided into the other units.

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Last updated 2 years ago