Python Cheat Sheet : The Basics

Data Types

String

Series of characters or data stored as text my_string = "Hello"

String Operations

returns the string with all uppercase letters
```
my_string.upper()
```
returns the length of a string
```
len(my_string)
```
returns the index of the first instance of the string inside the subject string, otherwise -1
```
my_string.find('l')
```
replaces any instance of the first string with the second in my_string
```
my_string.replace('H', 'C')
```

Integer

A whole number

my_integer = 12321

Float

A decimal number

my_decimal = 3.14

Boolean

Discrete value true or false

a = True
b = False

Dictionary

Changeable collection of key-value pairs

my_dictionary = {'banana': 1, 12: 'laptop', (0,0):'center'}

Dictionary Operations

Access value using key
```
my_dictionary['banana']
```
Get all keys in a dictionary as a list
```
my_dictionary.keys()
```
Get all values in a dictionary as a list
```
my_dictionary.values()
```

Tuple

Unchangeable collection of objects

tup = (1, 3.12, False, "Hi")

List

Changeable collection of objects

my_collection = [1, 1, 3.12, False, "Hi"]

List Operations

returns the length of a list
```
len(my_collection)
```
Add multiple items to a list
```
my_collection.extend(["More", "Items"])
```
Add a single item to a list
```
my_collection.append("Single")
```
Delete the object of a list at a specified index
```
del(my_collection[2])
```
Clone a list
```
clone = my_collection[:]
```

Concatenate two lists

my_collection_2 = ["a", "b", "c"]
my_collection_3 = my_collection + my_collection_2

Calculate the sum of a list of ints or floats

number_collection = [1,2,3,4.5]
sum(number_collection)

Check if an item is in a list, returns Boolean
```
item in my_collection
```
Check if an item is not in a list, returns Boolean
```
item not in my_collection
```

Set

Unordered collection of unique objects

a = {100, 3.12, False, "Bye"}
b = {100, 3.12, "Welcome"}

Set Operations

Convert a list to a set
```
my_set = set([1,1,2,3])
```
Add an item to the set
```
a.add(4)
```
Remove an item from a set
```
a.remove("Bye")
```
Returns set a minus b
```
a.difference(b)
```
Returns intersection of set a and b
```
a.intersection(b)
```
Returns the union of set a and b
```
a.union(b)
```
Returns True if a is a subset of b, false otherwise
```
a.issubset(b)
```
Returns True if a is a superset of b, false otherwise
```
a.issuperset(b)
```

Indexing

Accessing data from a string, list, or tuple using an element number

my_string[element_number]
my_collection[element_number]
my_tup[element_number]

Slicing

Accessing a subset of data from a string, list, or tuple using element numbers from start to stop -1

my_string[start:stop]
my_collection[start:stop]
my_tup[start:stop]

Comparison Operators

Comparison 0perators compare operands and return a result of true or false

Equal

a == b

Less Than

a < b

Greater Than

a > b

Greater Than or Equal

a >= b

Less Than or Equal

a <= b

Not Equal

a != b

Python Operators

+ : Addition
- : Subtraction
* : Multiplication
/ : Division
// : Integer Division (Result rounded to the nearest integer)

Conditional Operators

Conditional 0perators evaluate the operands and produce a true of false result

And

Returns true if both statement a and b are true, otherwise false

a and b

Or

Returns true if either statement a or b are true, otherwise false

a or b

Not

Returns the opposite of the statement

not a

Loops

For Loops

for x in range(x):

Executes loop x number of times

for x in iterable:

Executes loop for each object in an iterable like a string, tuple, list, or set

While Loops

while statement:

Executes the loop while statement is true

Conditional Statement

if statement_1:
    # Execute of statement_1 is true
if statement_2:
    # Execute if statement_1 is false and statement_2 is true
else:
    # Execute if all previous statements are false

Try/Except

try:
    # Code to try to execute
except a:
    # code to execute if there is an error of type a
except b:
    # code to execute if there is an error of type b
except:
    # code to execute if there is any exception that has not been handled
else:
    # code to execute if there is no exception

Error Types

IndexError - When an index is out of range
NameError - When a variable name is not found
SynntaxError - When there is an error with how the code is written
ZeroDivisionError - When your code tries to divide by zero

Range

Produce an iterable sequence from 0 to stop-1
```
range(stop)
```
Produce an iterable sequence from start to stop-1 incrementing by step
```
range(start, stop, step)
```

Webscraping

Import BeautifulSoup
```
from bs4 import BeautifulSoup
```
Parse HTML stored as a string
```
soup = BeautifulSoup(html, 'html5lib')
```
Returns formatted html
```
soup.prettify()
```
Find the first instance of an HTML tag
```
soup.find(tag)
```
Find all instances of an HTML tag
```
soup.find_all(tag)
```

Requests

Import the requests library
```
import requests
```

Send a get requests to the url optional parameters
```
response = requests.get(url, parameters)
```

Get the url of the response
```
response.url
```
Get the status code of the response
```
response.status_code
```
Get the headers of the request
```
response.request.headers
```
Get the body of the requests
```
response.request.body
```
Get the headers of the response
```
response.headers
```
Get the content of the response in text
```
response.text
```
Get the content of the response in json
```
response.json()
```

Send a post requests to the url optional parameters
```
requests.post(url, parameters)
```

Functions

Create a function

def function_name(optional_parameter_1, optional_parameter_2):
  # code to execute
  return optional_output

Calling a function

output = function_name(parameter_1, parameter_2)

Working with Files

Reading a File

Opens a file in read mode
```
file = open(file_name, "r")
```
Return the file name
```
file.name
```
Return the mode of file was opened in
```
file.mode
```
Reads the contents of a file
```
file.read()
```
Read a certain number of characters of a file
```
file.read(characters)
```
Read a single line of a file
```
file.readline()
```
Read all the lines of a file and stores it in a list
```
file.readlines()
```
Closes a file
```
file.close()
```

Writing to a File

Opens a file in write mode
```
file = open(file_name, "w")
```
Writes content to a file
```
file.write(content)
```
Adds content to the end of a file
```
file.append(content)
```

Objects and Classes

Creating a class

class class_name:
  def __init__(self. optional_parameter_1, optional_parameter_2)
      self.attribute_1 = optional_parameter_1
      self.attribute_2 = optional_parameter_2

  def method_name(self, optional_parameter_1):
      # Code to execute
      return optional_output

Create an instance of a class

object = class_name(parameter_1, parameter_2)

Calling an object method
```
object.method_name(parameter_3)
```

Files

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