python cheat sheet..
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Basics
Basic syntax from the python programming language
Showing Output To User
the print function is used to display or print output
print("Content that you wanna print on screen")
Taking Input From User
the input function is used to take input from the user
var1 = input("Enter your name: ")
Empty List
This method allows you to create an empty list
my_list = []
Empty Dictionary
By putting two curly braces, you can create a blank dictionary
my_dict = {}
Range Function
range function returns a sequence of numbers, eg, numbers starting from 0 to n-1 for range(0, n)
range(int_value)
Comments
Comments are used to make the code more understandable for programmers, and they are not
executed by compiler or interpreter.
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Single line comment
#This is a single line comment
Multi-line comment
'''This is a
multi-line
comment'''
Escape Sequence
An escape sequence is a sequence of characters; it doesn't represent itself when used inside
string literal or character.
Newline
Newline Character
\n
Backslash
It adds a backslash
\
Single Quote
It adds a single quotation mark
'
Tab
It gives a tab space
\t
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Backspace
It adds a backspace
\b
Octal value
It represents the value of an octal number
\ooo
Hex value
It represents the value of a hex number
\xhh
Carriage Return
Carriage return or \r is a unique feature of Python. \r will just work as you have shifted your
cursor to the beginning of the string or line.
\r
Strings
Python string is a sequence of characters, and each character can be individually accessed. Using
its index.
String
You can create Strings by enclosing text in both forms of quotes - single quotes or doublequotes.
variable_name = "String Data"
Slicing
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Slicing refers to obtaining a sub-string from the given string.
var_name[n : m]
String Methods isalnum() method
Returns True if all characters in the string are alphanumeric
string_variable.isalnum()
isalpha() method
Returns True if all characters in the string are alphabet
string_variable.isalpha()
isdecimal() method
Returns True if all characters in the string are decimals
string_variable.isdecimal()
isdigit() method
Returns True if all characters in the string are digits
string_variable.isdigit()
islower() method
Returns True if all characters in the string are lower case
string_variable.islower()
isspace() method
Returns True if all characters in the string are whitespaces
string_variable.isspace()
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isupper() method
Returns True if all characters in the string are upper case
string_variable.isupper()
lower() method
Converts a string into lower case
string_variable.lower()
upper() method
Converts a string into upper case
string_variable.upper()
strip() method
It removes leading and trailing spaces in the string
string_variable.strip()
List
A List in Python represents a list of comma-separated values of any data type between square
brackets.
List
var_name = [element1, element2, and so on]
List Methods index method
Returns the index of the first element with the specified value
list.index(element)
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append method
Adds an element at the end of the list
list.append(element)
extend method
Add the elements of a list (or any iterable) to the end of the current list
list.extend(iterable)
insert method
Adds an element at the specified position
list.insert(position, element)
pop method
Removes the element at the specified position and returns it
list.pop(position)
remove method
The remove( ) method removes the first occurrence of a given item from the list
list.remove(element)
clear method
Removes all the elements from the list
list.clear()
count method
Returns the number of elements with the specified value
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list.count(value)
reverse method
Reverse the order of the list
list.reverse()
sort method
Sorts the list
list.sort(reverse=True|False)
Tuples
Tuples are represented as a list of comma-separated values of any data type within parentheses.
Tuple Creation
variable_name = (element1, element2, ...)
Tuple Methods count method
It returns the number of times a specified value occurs in a tuple
tuple.count(value)
index method
It searches the tuple for a specified value and returns the position.
tuple.index(value)
Sets
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A set is a collection of multiple values which is both unordered and unindexed. It is written in
curly brackets.
Set Creation: Way 1
var_name = {element1, element2, ...}
Set Creation: Way 2
var_name = set([element1, element2, ...])
Set Methods: add() method
Adds an element to a set
set.add(element)
clear() method
Remove all elements from a set
set.clear()
discard() method
Removes the specified item from the set
set.discard(value)
intersection() method
Returns intersection of two or more sets
set.intersection(set1, set2 ... etc)
issubset() method
Checks if a Set is Subset of Another Set
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set.issubset(set)
pop() method
Removes an element from the set
set.pop()
remove() method
Removes the specified element from the Set
set.remove(item)
union() method
Returns the union of Sets
set.union(set1, set2...)
Dictionaries
The dictionary is an unordered set of comma-separated key: value pairs, within {}, with the
requirement that within a dictionary, no two keys can be the same.
Dictionary
= {: value, : value ...}
Adding Element to a dictionary
By this method, one can add new elements to the dictionary
[] =
Updating Element in a dictionary
If the specified key already exists, then its value will get updated
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[] =
Deleting Element from a dictionary
del let to delete specified key: value pair from the dictionary
del []
Dictionary Functions & Methods len() method
It returns the length of the dictionary, i.e., the count of elements (key: value pairs) in the
dictionary
len(dictionary)
clear() method
Removes all the elements from the dictionary
dictionary.clear()
get() method
Returns the value of the specified key
dictionary.get(keyname)
items() method
Returns a list containing a tuple for each key-value pair
dictionary.items()
keys() method
Returns a list containing the dictionary's keys
dictionary.keys()
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values() method
Returns a list of all the values in the dictionary
dictionary.values()
update() method
Updates the dictionary with the specified key-value pairs
dictionary.update(iterable)
Conditional Statements
The if statements are the conditional statements in Python, and these implement selection
constructs (decision constructs).
if Statement
if(conditional expression):
statements
if-else Statement
if(conditional expression):
statements
else:
statements
if-elif Statement
if (conditional expression) :
statements
elif (conditional expression) :
statements
else :
statements
Nested if-else Statement
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if (conditional expression):
if (conditional expression):
statements
else:
statements
else:
statements
Iterative Statements
An iteration statement, or loop, repeatedly executes a statement, known as the loop body, until
the controlling expression is false (0).
For Loop
The for loop of Python is designed to process the items of any sequence, such as a list or a
string, one by one.
for in :
statements_to_repeat
While Loop
A while loop is a conditional loop that will repeat the instructions within itself as long as a
conditional remains true.
while :
loop-body
Break Statement
The break statement enables a program to skip over a part of the code. A break statement
terminates the very loop it lies within.
for in :
statement1
if :
break
statement2
statement_after_loop
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Continue Statement
The continue statement skips the rest of the loop statements and causes the next iteration to
occur.
for in :
statement1
if :
continue
statement2
statement3
statement4
Functions
A function is a block of code that performs a specific task. You can pass parameters into a
function. It helps us to make our code more organized and manageable.
Function Definition
def my_function(parameters):
# Statements
File Handling
File handling refers to reading or writing data from files. Python provides some functions that
allow us to manipulate data in the files.
open() function
var_name = open("file name", "opening mode")
close() function
var_name.close()
Read () function
The read functions contains different methods, read(),readline() and readlines()
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read() #return one big string
It returns a list of lines
read-lines
It returns one line at a time
readline
Write () function
This function writes a sequence of strings to the file.
write () #Used to write a fixed sequence of characters to a file
It is used to write a list of strings
writelines()
Append () function
The append function is used to append to the file instead of overwriting it. To append to an
existing file, simply open the file in append mode (a):
file = open("Hello.txt", "a")
Exception Handling
An exception is an unusual condition that results in an interruption in the flow of the program.
try and except
A basic try-catch block in python. When the try block throws an error, the control goes to the
except block.
try:
[Statement body block]
raise Exception()
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except Exception as e:
[Error processing block]
OOPS
It is a programming approach that primarily focuses on using objects and classes. The objects
can be any real-world entities.
class
The syntax for writing a class in python
class class_name:
#Statements
class with a constructor
The syntax for writing a class with the constructor in python
# Default constructor
def __init__(self):
self.name = "CodeWithHarry"
# A method for printing data members
def print_me(self):
print(self.name)
object
Instantiating an object
= ()
filter function
The filter function allows you to process an iterable and extract those items that satisfy a given
condition
filter(function, iterable)
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issubclass function
Used to find whether a class is a subclass of a given class (classinfo) or not
issubclass(class, classinfo)
Iterators and Generators
Here are some of the advanced topics of the Python programming language like iterators and
generators
Iterator
Used to create an iterator over an iterable
iter_list = iter(['Harry', 'Aakash', 'Rohan'])
print(next(iter_list))
print(next(iter_list))
print(next(iter_list))
Generator
Used to generate values on the fly
# A simple generator function
def my_gen():
n = 1
print('This is printed first')
# Generator function contains yield statements
yield n
n += 1
print('This is printed second')
yield n
n += 1
print('This is printed at last')
yield n
Decorators
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Decorators are used to modifying the behavior of function or class. They are usually called before
the definition of a function you want to decorate.
property Decorator (getter)
@property
def name(self):
return self.__name
setter Decorator
It is used to set the property 'name'
@name.setter
def name(self, value):
self.__name=value
Deletor Decorator
It is used to delete the property 'name'
@name.deleter #property-name.deleter decorator
def name(self, value):
print('Deleting..')
del self.__name