Python cheatsheet for beginner

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Writing shorthand statements in python

Python is having shorthand statements and shorthand operators. These things will help you write more logic with less number of statements.

We will see those available shorthand statements.

File Example shorthand.py

lambda statement

Probably every body is aware of the lambda functions. The statement lambda is helpful to write single line functions with out naming a function. This will return the function reference where you can assign it to any arbitrary variable. It's more like JavaScript anonymous functions.

foo = lambda a: a+3
foo(3)
6
foo(8)
11

SELF CALLED LAMBDA

You can write the lambda and you can make it call it self like self-invoking functions in javascript. Let's see an example.

(lambda a: a+3)(8)
11
(lambda x: x**x)(3)
27

List Comprehension

List Comprehension is the great feature that python is having. Using this feature you can reduce the lot of code, you can reduces space complexity of the code. Simple for loops can be written using list comprehension.

Syntax:

L = [ mapping-expression for element in source-list if filter-expression ]

Where:

L Variable, result gets assigned to

mapping-expression Expression, which is executed on every loop if only filter-expression > in if condition resolved as True

This list comprehension is equivalent to.

result = []
for element in source-list:
  if filter-expression:
    result.append(mapping-expression)

EXAMPLE

Lets see list comprehension example. Get even number from the given range.

• Usual code

result = []
for i in range(10):
  if i%2 == 0:
    result.append(i)

print(result)
[0, 2, 4, 6, 8]

• List Comprehension

[i for i in range(10) if i%2==0]
[0, 2, 4, 6, 8]

Dict Comprehension

Dict comprehension is available in python 2.7 and 3.x. This syntax will provide you the way to encapsulate several lines you use to create dictionaries into one line. It's is similar to list comprehension but we use dict literals {} instead of [].

Syntax:

{key:value for element in source-list if filter-expression }

Let's how we use it by an example.

I have a list of fruits, I want to make it dictionary by changing their case

['APPLE', 'MANGO', 'ORANGE']

I want to convert all keys into lower case. This is we would do with out using comprehension.

l = ['MANGO', 'APPLE', 'ORANGE']

d = {}

for i in l:
  d[i.upper()] = 1

{'ORANGE': 1, 'MANGO': 1, 'APPLE': 1}

Using Simple list comprehension.

{i.upper(): 1 for i in l}

Set Comprehension

Set comprehension syntax is very much similar to dict comprehension with a small difference.

Let’s consider dict comprehension example. Using following statement you generate set

{i.upper() for i in l}

Where we haven’t specified value like we do in dict comprehension

Generator Expression

You might have already know about generators. Any function which contains yield statment is called generator. generator gives iterable where we can call next method to get the next item in the sequence. Python got short notation for this generators like lambda. It is same as list comprehension but we enclose the expression with touple literals instead.

• GENERATOR FUNCTION

def gen():
  for i in range(10):
    yield i 
g = gen()
<generator object gen at 0x7f60fa104410>
g.next()
0
g.next()
1

• GENERATOR EXPRESSION

Same generator function can written as follow.

g = (i for i in range(10))
g
<generator object <genexpr> at 0x7f60fa1045f0>
g.next()
0

Shorthand If Else

Like C and javascript ternary operator (?:) you can write short hand if-else comparison. By taking readability into account we have following syntax in python

if-expression if (condition) else else-expression

This is equivalent to.

if True:
  print("This is True")
else:
  print("This is False")

Tuple Unpacking

Python 3 even more powerful unpacking feature. Here it is.

Example:

a, rest = [1, 3, 4, 6]

In this case, a will get 1 and rest of the list will get assigned to variable rest. i.e [3, 4, 6]

String Concatenation with delimiter

If you want to concatenate list of strings with some random delimiter. You can do that by using string method join

" || ".join(["hello", "world", "how", "are", "you"])

'hello || world || how || are || you'

Powerful One-Liners

Are you tired of reading through lines of code and getting lost in conditional statements? Python one-liners might just be what you are looking for. For example, the conditional statements.

if alpha > 7:
   beta = 999
elif alpha == 7:
   beta = 99
else:
   beta = 0

can really be simplified to:

beta = 999 if alpha > 7 else 99 if alpha == 7 else 0

Removing duplicates items from a list

Most of the time we wanted to remove or find the duplicate item from the list. Let see how to delete duplicate from a list. The best approach is to convert a list into a set. Sets are unordered data-structure of unique values and don’t allow copies.

listNumbers = [20, 22, 24, 26, 28, 28, 20, 30, 24]
print("Original= ", listNumbers)

listNumbers = list(set(listNumbers))
print("After removing duplicate= ", listNumbers)

How to efficiently compare two unordered lists

Above two lists contains the same element only their order is different. Let see how we can find two lists are identical.

• We can use collections.Counter method if our object is hashable.
• We can use sorted() if objects are orderable.

from collections import Counter

one = [33, 22, 11, 44, 55]
two = [22, 11, 44, 55, 33]
print("is two list are b equal", Counter(one) == Counter(two))

Convert Byte to String

To convert byte to string we can decode the bytes object to produce a string. You can decode in the charset you want.

byteVar = b"pynative"
str = str(byteVar.decode("utf-8"))
print("Byte to string is" , str )

Convert hex string, String to int

hexNumber = "0xfde"
stringNumber="34"

print("Hext toint", int(hexNumber, 0))
print("String to int", int(stringNumber, 0))