python-hard-way/Codecademy.md

Codecademy's Python 2 Course

https://www.codecademy.com/courses/learn-python/lessons/python-syntax/

It's gratis and accepts Python 3 syntax.

Object-oriented

"The main goal of an object oriented language is to make code reusable – we do this through the use of classes and objects. If we want to design a new type of car, we can start with what they all have in common: wheels, seats, a frame. Now that we’ve determined what cars have in common, we can more easily implement any type of car we want by starting from that basic blueprint."

https://discuss.codecademy.com/t/what-does-it-mean-that-python-is-an-object-oriented-language/297314

Errors (ex6, CH1, P3)

"SyntaxError means there is something wrong with the way your program is written — punctuation that does not belong, a command where it is not expected, or a missing parenthesis can all trigger a SyntaxError.

A NameError occurs when the Python interpreter sees a word it does not recognize. Code that contains something that looks like a variable but was never defined will throw a NameError."

SyntaxError example: SyntaxError: EOL while scanning string literal

Math (ex6)

mirthful_addition = 12381 + 91817
amazing_subtraction = 981 - 312
trippy_multiplication = 38 * 902
happy_division = 540 / 45
sassy_combinations = 129 * 1345 + 120 / 6 - 12
exponents = (16 ** 0.5) # 16 to the 1/2th power. (4)
remainder = (15 % 2) # The remainder (and thus the result) equals 1

Find the remainder of a number using %

is_this_number_odd = 15 % 2
is_this_number_divisible_by_seven = 133 % 7

Updating variables / operators.

sandwich_price += sales_tax

is the same as:

sandwich_price = sandwich_price + sales_tax

but is much shorter.

Comments

Are indicated by # or """This is not for running"""

Numbers

An integer is like 5, a float is a number with a decimal point like 5.0. They can also be in scientific notation like 2.3e7

In Python 2, you need to make sure math like 7/2 = 3.5 is correct is by inputting it into Python like 7./2. or float(7)/2

Limitations of floats

Floats are limited by the number of digits. For example 1/3 = 0.3

>>> format(math.pi, '.12g')  # give 12 significant digits
'3.14159265359'

>>> format(math.pi, '.2f')   # give 2 digits after the point
'3.14'

Strings

Multi-line strings are marked by

Mulit-
line
strings"""

Booleans (True/False)

True = int(1) False = int(0)

Boolean Expressions

and evaluates true if both are true. or evaluates true if either are true. not return the opposite boolean value.

Relational Operators (ch.4, ex. 3)

== returns True if is is equal != returns True if is is NOT equal

Here are more of he same kind of operator:

> greater than < less than >= greater than or equal to <= less than or equal to

if, else, if else

elif = if else

Datatypes

Force treating as a string: str(7) Force treating as an integer: int("7") Force treating as a float: float(7)

Check Datatypes

Check datatypes using type(var)

Escaping Characters

Simply add a \ to escape a character that would otherwise cause issues.

Arrays / Indexes

cows = "cows"[0]

This sets the variable cows to the 0th letter of the string "cows" which is c. These indexes start at 0, not 1.

Strings

String Methods

len(var) Get length of string.

var.lower() Force lowercase

var.upper() Force uppercase

str(var) Force treating variable as a string.

If it uses dot notation like .lower(), it works exclusively on strings.

Concatenation

"Ten times a cow is equal to " + result + " with 10 times as many breeding opportunities."

or

print(var, var2, var3)

or

string1 += string2

String Formatting with %

"%s %s - 2020" % (month, day) # Replace %s with a variable. First the month, then the day.

Add %03d to specify a signed integer padded 2 places with zeros. For example, 2 becomes 02.

This is super useful for displaying dates like this: print("%02d-%02d-%02d") % (now.month, now.day, now.year) or time like this: print '%02d:%02d:%04d' % (now.hour, now.minute, now.second) (Ch3, Ex. 4)

Date and Time (Ch3)

Grab the current time:

from datetime import datetime
now = datetime.now()
year = now.year
month = now.month
day = now.day

Function P3 Ch.2

Defining a Function

  def greet_customer():
    print("Welcome!")

Calling Functions

```
greet_customer()
```

or if it has parameters:

greet_customer(1,ten)

Passing Arguments

greet_customer(special_item):
  print(special_item)

greet_customer(beef)

Result:

beef

Using Keyword Arguments

Keyword arguments are nice for specifying a default but changeable argument.

Here's an example from P3, Ch2, ex7

def create_spreadsheet(title, row_count = 1000):
  row_count = str(row_count)
  print("Creating a spreadsheet called " + title + " with " + row_count +" rows.")

create_spreadsheet("Applications", row_count = 10)

row_count = 1000

is the default

row_count = 10

is the passed argument and thus what is used for a result:

Creating a spreadsheet called Applications with 10 rows.

Returning Stuff

You can return stuff like this to store for later:

def addfour(number, cow):
  addedfour = number + 4
  cat = number - 4
  return addedfour, cat # All returned arguments must be on the same return call.

I'll make it add four to 456

yo, cow = addfour(456)
print ("456 + 4 equals " + str(yo) )

460

You can also do this with multiple arguments:

x_squared, y_squared = square_point(1, 3)

Lists

You can put either strings, integers or other lists in a list, probably other things too.

heights = [['Jenny', 61], ['Alexus', 70], ['Sam', 67], ['Grace', 64]]

Use zip to combine elements from two lists: zip(names, dogs_names) would match dogs and their owners.

But don't forget to turn it into a list before printing print(list(zipvar))

You can append values to lists like this:

orders = ['daisies', 'periwinkle']
orders.append('tulips')

Ranges

You can get a range of numbers using range()

It will give you all numbers under the number you input. For example, range(8) would give you 0-7, range(1, 8) would give you 1-7, and range(1, 8, 2) would give you 1,3,5,7 (2 is the interval) Use print(list(var)) when printing.

length

Grab length with len(list)

Selecting List elements

Use: listname[indexnumber]

The index starts at 0. Grab the last in an index using list[-1]

Giving values to list elements

lst[index] = indexlst

Cutting the middle of a list (ex3, functions + lists, ch.4)

def remove_middle(lst, start, end):
end = end + 1
return lst[:start] + lst[end:]

print(remove_middle([4, 8, 15, 16, 23, 42], 1, 3))

This prints [4, 23, 42] removing the index 1-3.

If something occurs more than N times, return True

def more_than_n(lst, item, n):
if lst.count(item) > n:
  return True
else:
  return False

  print(more_than_n([2, 4, 6, 2, 3, 2, 1, 2], 2, 3))

Sublists

Grab a subset of a list using sublist = letters[1:6] This would give you index 1-5.

You can also do [:5] for all up to index 4, and [5:] for all after index 5. And, you can do [-3:] for the last 3 in an index.

Counting frequency of elements in a list.

var.count('stringtofind')

Sorting strings alphabetically

var.sort()
print(var)

or use

sortedvar = sorted(var)

to produce a new list with sorted contents without changing the original variable.

Sorting strings based on an element in a sublist

Replace 1 with the index you want to sort by.

`pizzas.sort(key=lambda x: x[1])``

Catch Errors

try:
  command_may_cause_error
except NameOfError:
  print("Nice little message to user.")

Fun Projects

Design a shop using Ex7 and Ex9 as a frame:

7:

money_in_wallet = 40
sandwich_price = 7.50
sales_tax = .08 * sandwich_price

sandwich_price += sales_tax
money_in_wallet -= sandwich_price

9:

cucumbers = 1
price_per_cucumber = 3.25
total_cost = cucumbers * price_per_cucumber
print(total_cost)

total_price += nice_sweater

Cool concept from Ch2 Ex15:

name = raw_input("What is your name? ")
quest = raw_input("What is your quest? ")
color = raw_input("What is your favorite color? ")

print "Ah, so your name is %s, your quest is %s, " \
"and your favorite color is %s." % (name, quest, color)