Python Basics
Single Line Comments |
#This is a comment on a single line |
Doc String: These appear right after a function definition |
Def foo:
‘’’this is a docstring, this will usually contain information about what is within the function’’’ |
Arithmetic Operations:
+ is addition
- is subtraction
* is multiplication
/ is division
% is modulus (remainder)
** is exponentiation |
Addition: result = 1 + 3
Subtraction: result = 1 - 3
Multiplication: result = 1 * 3
Division: result = 1 / 3
Modulus: result = 1 % 3
Exponentiation: result = 1 ** 3 |
Plus-Equals Operator += |
counter = 0
counter += 10
This is Equivalent to:
counter = 0
counter = counter + 10
**Note: This also works with strings |
Variables: Unlike in many other programming languages, python does not require you to declare a type before assigning a value to the variable |
user_name = ‘Randomgirlll13’
id_number = 2345
user_verified = True
user_float = 14
user_float = 14.55 |
String Concatenation |
first = ‘good’
second = ‘morning’
sentence = first + last + ‘!’ |
Function print(): This outputs information to the user in the format of text |
print(‘Hello World!’)
print(100 +300)
print(14.555556) |
Loops
Break Keyword |
In a loop, the break keyword will escape the loop
#Example
nums = [0, 2, -3, 5, 7]
for num in nums:
if (num < 0):
break
this will only run to -3 before breaking out of the loop |
List Comprehension |
This is a concise way of creating lists.
Syntax: list_name = [Expression for Item in List <if Confiditonal>]
The expressions can be anything.
A list comprehension will ALWAYS return a list |
For Loops |
#Example
nums = [1,2,3,4,5]
for num in nums:
print(num) |
Continue Keyword |
The continue Keyword is inside a loop to skip the remaining code within the loop and begin the next loop iteration |
Loops with range() function |
Using the range() function, we can have a for loop that performs an action a specific number of times
#Example
for i in range(3):
print(i) #Prints 0, 1, 2 |
While Loops |
A while loop will repeatedly execute a code block as long as the condition is True
hungry = True
while hungry:
print(‘I’m Hungry)
hungry = False |
Modules
Importing Python Modules |
The keyword import can be used to import python modules.
#Example
import module
module.function() |
Module importing from file |
To import from a file, provided it is in the same folder as the current file you are writing, you can import it as follows
import filename |
Aliasing with ‘as’ Keyword |
The ‘as’ keyword can give an alias to a python module or function
#example
from matplotlib import pyplot as plt
plt.plot(x,y) |
Random Module |
The random module offers methods that simulate non-deterministic behavior in selecting numbers from a range |
Files
Python File Object |
A python file object is created with the open() function. You can associate the file object with a variable using the with and as keywords
with open('somefile.txt') as file_object: |
Python Read Method |
After having opened a file with open(), call the .read() method to return the entire file contents as a Python string. |
Python Readline Method |
If you only want to read one line, use Readline() on the file object. This will extract one single line of text at a time |
Python Readlines Method |
Instead of getting a single string of text, readlines will return a list of strings representing individual lines in the file |
Python Write to File |
By default, all files opened are only for reading. To write to a file, you must open the file with a 'w' argument, then you can use the .write() method to write the file.
**Note If the file already exists, all prior content will be overwritten.
Example
with open('text.txt', 'w') as text:
text.write('This is example text') |
Python Append to File |
Since writing to an existing file will overwrite it, to keep the original contents, we can write to a file using append instead. To appead we pass it an 'a' argument in place of a 'w' |
Class csv.DictWriter |
the csv module implements classes to read and write data in CSV format.
This has a class DictWrite which operates like a normal writer but will map a dictionary onto output rows. The keys of the dictionary are column names while values are actual data.
csv.DictWriter constructor takes two arguments. first is the open file handler that CSV is written to. second is 'fieldnames', this is a list of field names that the CSV is going to handle. |
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Control Flow Operations
Else If Statements |
#elif Statement
pet_type = ‘fish’
if pet_type == ‘dog’:
print(“You have a dog.”)
elif pet_type == ‘fish’:
print(“You have a fish.”)
else:
print(“Not Sure!”) |
Or Operator |
True or True #Evaluates to True
True or False #Evaluates to True
False or False #Evaluates to False
1 < 2 or 3 < 1 #Evaluates to True |
Equal Operator |
==
Used to compare two values, variables or expressions to determine if they are the same.
if they are the same, it returns True. Otherwise, it will return False |
Not Equals Operator |
!=
This is used to compare two values, variables or expressions to see if they are not the same.
If they are not the same, it returns True. Otherwise, it will return False |
Comparison Operators |
< #Less than
> #Greater than
<= #less than or equal to
>= #Greater than or equal to |
And Operator |
True and True #Evaluates to True
True and False #Evaluates to False
False and False #Evaluates to False
1 == 1 and 1 < 2 #Evaluates to True |
Not Operator |
not True #Evaluates to False
not False #Evaluates to True
not 1 > 2 #Evaluates to True |
Functions
Functions |
If a task will need to be performed multiple times, it is good practice to have it done within a function.
In python these are defined with the ‘def’ keyword and then the name of your function
#Example
def my_function(x): |
Function Parameters |
Some functions require input to provide data to their code. These are known as parameters. A function can have no parameters, one parameter, or multiple parameters
#Examples
def zero_function():
def one_function(number):
def three_function(age, height, weight): |
Calling Functions |
To call a function you simply have the name of the function and the arguments it needs
#Example
zero_function()
one_function(44) |
Variable Scope |
When it comes to the scope of variables, those not within a function are typically global variables, those within a function are local variables to that function and can be utilized in that function only.
If you want to get a value back from a function you can use the special keyword return |
Dictionaries
Syntax of Dictionaries in Python |
example_dictionary = {"elem1": 1, "elem2": 2} |
Dictionary Value Types |
In Python, the 'Value' type can be anything, the 'key' type must be a mutable data type |
Accessing and Writing data in a dictionary |
Values can be accessed by placing the key within square brackets next to the dictionary name
print(example_dictionary["elem1"]
To write a new value to a key it is the same syntax as accessing but with an = sign and what you'd like the new value to be
example_dictionary["elem1"] = 3 |
Merging Dictionaries with .update() |
If two dictionaries need to be combined you can use the .update() function
dict1 = {1 : 'one'}
dict2 = {2 : "two"}
dict1.update(dict2) {{n1}} //dict1 is now {1 : 'one", 2 : 'two"} |
Dictionary key-value Methods |
if you want to look the keys, values, or both of the dictionary, there are methods
.keys() will return a list of the keys
.values() will return a list of the values
.items will return a list of tuples containing the key -value pairs |
Dictionary get() Method |
The get() method will return the value of a key if it exists otherwise it will return None if no default value is given for the key |
Dictionary .pop() Method |
.pop() will remove a key from a dictionary and return that keys value |
Classes
Instantiate Python Class |
class Example:
""This is an empty class""
pass |
Python Class Variable |
Class variables are defined locally within the class and outside of all methods. They have the same value for every instance of the class
they can be access with instance.variable or class_name.variable syntax |
Python repr Method |
The Python __repr__() method is used to tell Python what the string representation of the class should be. It only has one parameter, self, and it returns a string |
Python Class Methods |
In Python, methods are functions that are defined as part of a class. Common pract is that the first argument of any method that is part of a class is the actual object calling the method. This argument is usually called self |
Python init Method |
In Python, the __init__ method is used to initalize a newly created object. It will be called every time the class is instantiated
class Animal:
def __init__ (self, voice):
self.voice = voice
cat = Animal('Meow') |
Python type() function |
the type() function will return the data type of the argument that was passed to it |
Python dir() function |
In Python, the dir() function, with no arguments, returns a list of all the attributes in current scope
With an object as argument, dir() will try tor return all valid object attributes |
_main_ in Python |
In Python, __main__ is an identified used to reference the current file context |
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Lists
List Syntax |
primes = [1,2,3,5,7,11]
empty_list = [] |
Adding Lists Together |
items = [‘cake’, ‘cookie’, ‘pie’]
total_items = items + [‘tart’, ‘cheesecake’]
print(total_items) #Result: [‘cake’, ‘cookie’, ‘pie’, ‘tart’, ‘cheesecake’] |
Lists: Data Types |
Lists can contain multiple types of data types within one list |
List Method .append() |
numbers = [11, 333, 44]
numbers.append(22)
print(numbers)
#Result: [11, 333, 44, 22] |
List Indexing |
In Python, Lists start at zero for the first index spot
#Example
names = [‘Lauren’, ‘Maria’, ‘Bailey’]
‘Maria’ is in the first index spot and ‘Lauren’ is in the zero index spot |
Negative List Indexing |
In Python, You can also access list elements using negative indices.
#Example
names = [‘Kim’, ‘Ashley’, ‘Hailey’, ‘Ginny’]
names[-1] # ‘Ginny’
names [-4] # ‘Kim’ |
List Method .remove() |
This will remove the first occurrence of an element from a list in python |
List Method .count() |
This will return the number of times a certain element shows up in the list |
Determining List Length |
The len() function can be used to determine the number of items found in a list
#Example
sack = [2, 4, 5, 6]
size = len(sack)
print(sack) # 4 |
List Method .sort() |
This Method will sort the contents of the list in either ascending order (numerical lists), or alphabetical order (string lists) |
List Slicing |
This allows for only a portion of the list to be returned
#example
tools = [‘hammer’, ‘ruler’, ‘pen’]
tools_slice = tools[1:3] # [‘ruler’, ‘pen’]
**Note: The original list will remain unaltered |
Sorted() Function |
This will take a list as the functions argument and will return a new sorted list without altering the original list |
List Method .insert() |
This allows us to add an element to a specific index into the list |
List Method .pop() |
This allows us to remove an element from the list and also return it |
Strings
Escaping Characters |
Backslashes (\) are used to escape characters in Python Strings |
In Syntax |
The in syntax is used to determine if a letter or substring exists within a string. This will return True or False
#Example
sentence = “Creating this has been a lot of work”
print(“work” in sentence) #True |
Indexing and Slicing Strings |
Using the same notation as lists, you can index strings
You can also get a substring from a string using slicing, the notation is string_name[start:end] |
Iterate Strings |
To iterate through a string, utilize the for … in notation
#Example
str = “hello”
for c in str:
print(c) |
String len() function |
This function can be used to determine the length of a string among other objects |
String Concatenation |
To combine two strings, simply use the + operator |
String Immutability |
In python, strings are considered immutable, meaning once it has been defined, it cannot be changed |
String .format() |
This replaces empty braces ({}) placeholders in a string with the arguments passed.
If keywords are specified within the placeholders, they are replaced with the corresponding named arguments
#Example
msg1 = ‘Mary had {} glasses of water and John had {} glasses of juice.’
msg1.format(3, 2) |
String Method .lower() |
This will convert a string to all lowercase letters |
String Method .strip() |
This will remove characters from the beginning and end of a string. You can specify what characters to remove |
String Method .title() |
This will return a string in title case |
String Method .split() |
This will split a string into a list of items based on arguments.
If no arguments are passed, it uses white space, otherwise it will split based on whatever the argument passed is |
String Method .find() |
This will return the index of the first occurence of the string argument passed. If nothing is found, it will return -1 |
String Method .replace() |
This will replace the first occurrence of the first string argument with the second string argument |
String Method .upper() |
This will make the string all uppercase |
String Method .join() |
This concatenation a list of strings together with the desired delimiter |
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