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awesome-cheatsheets/languages/python.md
2021-10-08 11:11:28 +08:00

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Python

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

  • It is also Object oriented, modular oriented and a 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 flower 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
  2. Create the program: nano/cat > nameProgram.py
  3. Write the program and save it
  4. python nameProgram.py

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]

Keywords


  • 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
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

Operators


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
&&, 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:

thislist = ["apple", "banana", "cherry"] 
  • List items are ordered, changeable, and allow duplicate values.

  • List items are indexed, the first item has index [0], the second item has index [1] etc.

  • The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.

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

  • A list can contain different data types:

list1 = ["abc", 34, True, 40, "male"]
  • It is also possible to use the list() constructor when creating a new list
thislist = list(("apple", "banana", "cherry"))  # note the double round-brackets
  • pop() function removes the last value in the given list by default.

    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.
thistuple = ("apple", "banana", "cherry")
  • Tuple items are ordered, unchangeable, and allow duplicate values.

  • 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.

  • Tuples are unchangeable, meaning that we cannot change, add or remove items after the tuple has been created.

  • Since tuple are indexed, tuples can have items with the same value:

  • Tuples allow duplicate values:

thistuple = ("apple", "banana", "cherry", "apple", "cherry")
  • To determine how many items a tuple has, use the len()function:
thistuple = ("apple", "banana", "cherry")
print(len(thistuple))
  • 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.
thistuple = ("apple",)
print(type(thistuple))

# NOT a tuple
thistuple = ("apple")
print(type(thistuple))
  • It is also possible to use the tuple() constructor to make a tuple.

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

Set

  • Set is a collection which is unordered and unindexed. No duplicate members.
  • A set is a collection which is both unordered and unindexed.
thisset = {"apple", "banana", "cherry"}
  • Set items are unordered, unchangeable, and do not allow duplicate values.

  • Unordered means that 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.

  • Sets are unchangeable, meaning that we cannot change the items after the set has been created.

  • Duplicate values will be ignored.

  • To determine how many items a set has, use the len() method.

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

print(len(thisset))
  • Set items can be of any data type:
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.
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.
set1 = {"apple", "banana", "cherry"}
frzset=frozenset(set1)
print(frzset)

Dictionary

  • Dictionary is a collection which is unordered and changeable. No duplicate members.
  • Dictionaries are used to store data values in key:value pairs.
  • Dictionaries are written with curly brackets, and have keys and values:
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.
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict["brand"])
  • Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.
  • Dictionaries cannot have two items with the same key.
  • Duplicate values will overwrite existing values.
  • To determine how many items a dictionary has, use the len() function.
print(len(thisdict))
  • The values in dictionary items can be of any data type
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

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

Conditional branching

    if condition:
        pass
    elif condition2:
        pass
    else:
        pass

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
i = 1
while i < 6:
  print(i)
  i += 1
  • 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.

  • With the break statement we can stop the loop even if the while condition is true

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

  • With the else statement we can run a block of code once when the condition no longer is true.

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.

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)
  • 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 sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

  • The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3).

  • 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":

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.
for x in [0, 1, 2]:
  pass

Function definition

def function_name():
    return

Function call

function_name()
  • We need not to specify the return type of the function.
  • Functions by default return None
  • We can return any datatype.