awesome-cheatsheets/languages/python.md

# Python

* Python is an interpreted, high-level and general-purpose, dynamically typed programming language

* It is seen as an object-oriented, modular scripting language.

* In Python, everything is considered as an object.

* A python file has an extension of .py

* Python follows indentation to separate code blocks instead of using flower/curly brackets ({}).

* We can run a python file by the following command in cmd(Windows) or shell(mac/linux).

    `$ python <filename.py>` or `$ python3 <filename.py>`

#### By default, python doesn't require any imports to run a python file.

## Create and execute a program

1. Open up a terminal/cmd
1. Create the program: nano/cat > nameProgram.py
1. Write the program and save it
1. python nameProgram.py

<br>

### Basic Datatypes

| Data Type | Description |
| --------- | ----------- |
| int | Integer values [0, 1, -2, 3] |
| float | Floating point values [0.1, 4.532, -5.092] |
| char | Characters [a, b, @, !, `] |
| str | Strings [abc, AbC, A@B, sd!, `asa] |
| bool | Boolean Values [True, False] |
| complex | Complex numbers [2+3j, 4-1j] |

<br>

## Keywords
<br>

- As of python3.8 there are 35 keywords

| Keyword | Description  | Category |
|---------- | ---------- | --------- | 
| True      | Boolean value for not False or 1 | Value Keyword|
| False     | Boolean Value for not True or 0 | Value Keyword |
| None      | No Value | Value keyword |
| and       | returns true if both (oprand) are true (other language && ) | Operator keyword |
| or        | returns true of either operands is true (other language || ) | Operator keyword |
| in        | returns true if word is in iterator | Operator keyword |
| is        | returns true if id of variables are same | Operator keyword |
| not       | returns opposite Boolean value | Operator Keyword |
| if | get into block if expression is true | conditional |
| elif | for more than 1 if checks | conditional |
| else | this block will be executed if condition is false | conditional |
| for | used for looping | iteration |
| while | used for looping | iteration |
| break | get out of loop | iteration | 
| continue | skip for specific condition | iteration |
| def | make user defined function | structure |
| class | make user defined classes | structure |
| lambda | make anonymous function | structure |
| with | execute code within context manager's scope | structure |
| as | alias for something | structure |
| pass | used for making empty structures(declaration) | structure |
| return | get value(s) from function, get out of function | returning keyword |
| yield | yields values instead of returning (are called generators) | returning keyword |
| import | import libraries/modules/packages | import |
| from | import specific function/classes from modules/packages | import |
| try | this block will be tried to get executed | exception handling |
| except | is any exception/error has occured it'll be executed | exception handling |
| finally | It'll be executed no matter exception occurs or not | exception handling | 
| raise | throws any specific error/exception | exception handling |
| assert | throws an AssertionError if condition is false | exception handling |
| async | used to define asynchronous functions/co-routines | asynchronous programming |
| await | used to specify a point when control is taken back | asynchronous programming |
| del | deletes/unsets any user defined data |  variable handling |
| global | used to access variables defined outside of function | variable handling |
| nonlocal | modify variables from different scopes | variable handling |
<br>

## Operators

<br>

| Operator | Description |
|-|-|
|  ( )	|  grouping parenthesis, function call, tuple declaration |
|  [ ]	|  array indexing, also declaring lists etc.|
|  !	|    relational not, complement, ! a  yields true or false |
|  ~   | 	bitwise not, ones complement, ~a |
| \-   |	unary minus, - a |
|  \+   | 	unary plus,  + a |
|  \*   |	multiply, a * b |
|  /   	| divide, a / b |
|  %    |	modulo, a % b |
|  \+   | 	add, a + b |
| \-   | 	subtract, a - b |
| <<   | shift left,  left operand is shifted left by right operand bits (multiply by 2) |
| \>>   |	shift right, left operand is shifted right by right operand bits (divide by 2) |
 | <    |	less than, result is true or false,  a %lt; b
| <=   |	less than or equal, result is true or false,  a <= b
| \>    |	greater than, result is true or false,  a > b
| \>=   |	greater than or equal, result is true or false, a >= b
|  ==   |	equal, result is true or false,  a == b
| !=  | 	not equal, result is true or false,  a != b
|  & | bitwise and,  a & b
| ^ | bitwise exclusive or XOR,  a ^ b
| \| | bitwise or,  a | b
|  &&, and | relational and, result is true or false,  a < b && c >= d
| \|\|, or | relational or, result is true or false,  a < b \|\| c >= d |
| =  | store or assignment |
|  += | add and store |
|  -=  | subtract and store |
|  *= | multiply and store |
|  /= | divide and store|
|  %= | modulo and store|
| <<= | shift left and store|
|  \>>= | shift right and store|
|  &= | bitwise and and store|
|  ^= | bitwise exclusive or and store|
|  \|= | bitwise or and store|
|  , | separator as in   ( y=x,z=++x )|

### Basic Data Structures

### List

- List is a collection which is ordered and changeable. Allows duplicate members.


- Lists are created using square brackets:

```python
thislist = ["apple", "banana", "cherry"] 
```

- List items are ordered, changeable / mutable, and allow duplicate values. We can change, add, and remove items in a list after it has been created.

- List items are zero-indexed. The first item has index `[0]`, the second item has index `[1]` etc.

- To determine how many items a list has, use the `len()` function.

- A list can contain different data types:
```python
list1 = ["abc", 34, True, 40, "male"]
```
- Lists can either be created as above or using the list() constructor as follows:
```python
thislist = list(("apple", "banana", "cherry"))  # note the double round-brackets
```

- pop() function removes and returns the last value in the given list by default. Should a different index be added in the brackets, the element with that index will be removed and returned.

  ```python
  thislist = ["apple", "banana", "cherry"] 
  
  print(thislist.pop())  # cherry
  print(thislist.pop(0))  # apple 
  
  ```

  

### Tuple

- Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
- A tuple is a collection which is ordered and unchangeable.
- Tuples are written with round brackets.
```python
thistuple = ("apple", "banana", "cherry")
```

- Tuple items are ordered, unchangeable / immutable, and allow duplicate values. We cannot change, add or remove items after the tuple has been created.
- Tuple items are indexed, the first item has index `[0]`, the second item has index `[1]` etc.
- When we say that tuples are ordered, it means that the items have a defined order, and that order will not change.

- Since tuple are indexed, tuples can have items with the same value:
- Tuples allow duplicate values:
```python
thistuple = ("apple", "banana", "cherry", "apple", "cherry")
```
- To determine how many items a tuple has, use the  `len()`function:
```python
thistuple = ("apple", "banana", "cherry")
print(len(thistuple)) # 3
```
- To create a tuple with only one item, you have to *add a comma after the item*, otherwise Python will not recognize it as a tuple.
```python
thistuple = ("apple",)
print(type(thistuple)) # tuple

# NOT a tuple
thistuple = ("apple")
print(type(thistuple)) # string
```
- It is also possible to use the tuple() constructor to make a tuple:
```python

thistuple = tuple(("apple", "banana", "cherry")) # note the double round-brackets
print(thistuple)
```

### Set
- Set is a collection which is unordered and unindexed. There can be no duplicate members.
- Since sets are unordered and unindexed, the items in a set do not have a defined order. Set items can appear in a different order every time you use them, and cannot be referred to by index or key.
- Duplicate values will be ignored.

```python
thisset = {"apple", "banana", "cherry"}
```

- To determine how many items a set has, use the `len()` method.
```python
thisset = {"apple", "banana", "cherry"}

print(len(thisset)) # 3
```

- Set items can be of any data type:
```python
set1 = {"apple", "banana", "cherry"}
set2 = {1, 5, 7, 9, 3}
set3 = {True, False, False}
set4 = {"abc", 34, True, 40, "male"}
```

- It is also possible to use the `set()` constructor to make a set:
```python
thisset = set(("apple", "banana", "cherry")) # note the double round-brackets
```

- frozenset()  is just an immutable version of Set. While elements of a set can be modified at any time, elements of the frozen set remain the same after creation.

```python
set1 = {"apple", "banana", "cherry"}
frzset=frozenset(set1)
print(frzset) # {"apple", "banana", "cherry"}
```



### Dictionary

- Dictionary is a collection which is unordered and changeable. There can be no duplicate members.
- Dictionaries are used to store data values in key:value pairs.
- Dictionaries cannot have two items with the same key.
- Dictionaries are written with curly brackets, and have keys and values:

```python
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
```

- Dictionary items are presented in key:value pairs, and can be referred to by using the key name.
```python
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict["brand"]) # Ford
```

- Dictionaries are changeable / mutable, meaning that we can change, add or remove items after the dictionary has been created.
- Duplicate values will overwrite existing values.
- To determine how many items a dictionary has, use the `len()` function.
- 
```python
print(len(thisdict)) # 3
```

- The values in dictionary items can be of any data type
```python
thisdict = {
  "brand": "Ford",
  "electric": False,
  "year": 1964,
  "colors": ["red", "white", "blue"]
}
```

- pop() Function is used to remove a specific value from a dictionary. You can only use key bot the value. Unlike Lists you have to give a value to this function

  ```python
   car = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
  }
  
  x = car.pop("model")
  
  print(x) # Mustang
  print(car) # {'brand': 'Ford', 'year': 1964}
  ```

  

### Conditional branching

```python
    if condition:
        pass # Code to execute if condition is true.
    elif condition2:
        pass # Code to execute if condition2 is true.
    else:
        pass # Code to execute if neither condition nor condition2 is true.
```

### Loops

 Python has two primitive loop commands:
1. while loops
2. for loops

#### While loop
- With the `while` loop we can execute a set of statements as long as a condition is true.

- Example: Print i as long as i is less than 6
```python
i = 1
while i < 6:
  print(i)
  i += 1
  
# Output: 1, 2, 3, 4, 5
```

- The while loop requires relevant variables to be ready. In this example we need to define an indexing variable, i, which we set to 1. This has to be done before starting with the loop.

- With the `break` statement we can stop the loop even if the while condition is true.

- Example: Print i as long as i is less than 6
```python
i = 1
while True:
    if i < 6:
        print(i)
        i += 1
    else:
        break
# Output: 1, 2, 3, 4, 5
```

- With the `continue` statement we can stop the current iteration, and continue with the next.

- Example: Print i as long as it is less than 6 and not equal to 3.
```python
i = 0
while i < 6:
  i += 1
  if i == 3:
    continue
  else:
    print(i)
# Output: 1, 2, 4, 5
```

- With the `else` statement we can run a block of code once when the condition no longer is true.
```python
i = 1
while i < 6:
  print(i)
  i += 1
else:
  print("The loop is finished")
  
# Output: 1, 2, 3, 4, 5, The loop is finished
```

#### For loop
- A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

- This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

- With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.
```python
fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x) # apple banana cherry
```
- The for loop does not require an indexing variable to set beforehand.

- To loop through a set of code a specified number of times, we can use the range() function. 
- The range function returns a list of integers starting at zero or the starting value up to but not including the ending value, in increments of 1 unless otherwise defined:

```python
range(4) # [0, 1, 2, 3]
range(2, 4) # [2, 3]
range(1, 10, 2) # [1, 3, 5, 7, 9]
```

- The else keyword in a for loop specifies a block of code to be executed when the loop is finished.

- A nested loop is a loop inside a loop. The "inner loop" will be executed one time for each iteration of the "outer loop":

```python
adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
  for y in fruits:
    print(x, y)
```

- for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error.

```python
for x in [0, 1, 2]:
  pass
```

### Function definition
```python
def function_name():
    return
```
### Function call

```python
function_name()
```

* A function must either return a value or have some content.
* We need not to specify the return type of the function.
* Functions by default return `None` 
* We can return any datatype.