Arindam Dawn

Posted on Jun 25, 2020 • Edited on Jun 30, 2020 • Originally published at tabandspace.com

30 Days of Python 👨‍💻 - Day 5 - Conditions & Loops I

#python #challenge

It has been an exciting 4 days of experience learning Python. Until now I have been able to cover some of the basic syntaxes of Python along with the data types and how to perform actions on them using built-in functions and methods along with some best practices. This was probably the dull parts and scratching the surface of Python. Today my short goal was to understand logical and conditional programming along with repeating tasks using loops in Python. Very interesting indeed!

Conditional Logic

age = input('Please enter your age')
if(int(age) >= 18):
  print('You are allowed to enter the club')
else:
  print('Sorry! you are not allowed!')

The above is an example of an if-else conditional statement in Python. It is used to execute logic based on some condition. If the condition is not fulfilled, then the code in the else block is executed. Coming from the JavaScript universe, the differences I noted is there are no curly braces surrounding the if-else blocks and instead of a : is used after the conditional statement. The block inside a conditional statement is also indented. I wired these two things to my Python mental model.

exam_score = input('Enter your exam score')
if(int(exam_score) > 90):
  print('You have got grade A+')
elif(int(exam_score) > 80):
  print('You have got grade A')
else:
  print('You have got grade B')

If there are multiple conditions that need to be executed, then an elif block is used. In many other programming languages including JavaScript, it is called else if block. There can be any number of else if blocks to check different conditions. Python makes it more compact.

is_adult = True
is_licensed = True

if(is_adult and is_licensed): ## In JavaScript && is used instead of 'and'
  print('You are allowed to drive')
else:
  print('You are not allowed to drive')

The above code checks for two conditions in a single expression to execute the block. The and keyword is used to check if both conditions evaluates to True then only the block is executed. This syntax is so easy to read! Python definitely is more readable.

Indentation

Indentation is the way to separate lines of code using spaces/tabs so that the interpreter can distinguish code blocks and execute the code accordingly. Python does not use any braces unlike JavaScript and instead uses indentation to chunk blocks of code. The code editors make our life easy by automatically doing the indentation. The online editor Repl that I am using as a playground also does that.

if(10 > 8):
  print('Such a silly logic. I will get printed')
else:
  print('I will never get printed')
  print('I am not get printed coz I am indented')

# Now without indentation
if(10 > 8):
  print('Such a silly logic. I will get printed')
else:
  print('I will never get printed')
print('I will be printed anyways coz I am not indented')
# The above line gets printed always as interpreter treats it as a new line

Truthy and Falsy

Values that evaluate to true are called Truthy
Values that evaluate to false are called Falsy

When checking for a condition, the expression inside a condition is evaluated to a boolean value using type conversion. Now all values are considered "truthy" except for the following, which are "falsy":

None
False
0
0.0
0j
Decimal(0)
Fraction(0, 1)
[] - an empty list
{} - an empty dict
() - an empty tuple
'' - an empty str
b'' - an empty bytes
set() - an empty set
an empty range, like range(0)
objects for which
- obj.__bool__() returns False
- obj.__len__() returns 0

username = 'santa' # bool('santa') => True
password = 'superSecretPassword' # bool('superSecretPassword') => True
if username and password:
  print('Details found')
else:
  print('Not found')

Ternary Operator

While going through the syntax of the ternary operator in Python, I found it a bit confusing initially but then found it quite easy to read compared to the one I am familiar with in my JavaScript world

is_single = True
message = 'You can date' if is_single else 'you cannot date'
# result = (value 1) if (condition is truthy) else (value 2)
print(message) # You can date

Ternary operator is also sometimes called a conditional expression. It is a very handy way to checking a condition in a single statement. I compared it with the ? operator syntax of JavaScript to solidify my mental model.

Short-Circuiting

While checking for multiple logical conditions in a single statement, the interpreter is smart enough to to ignore the rest of the condition check if the first condition fails. This is known as short-circuiting. It can be better explained using an example

knows_javascript = True
knows_python = True

if(knows_javascript or knows_python): # doesn't depend on value of knows_python
  print('Javscript or python developer')
else:
  print('Some other developer')

knows_javascript = False
knows_python = True

if(knows_javascript and knows_python): # doesn't depend on value of knows_python
  print('Javscript or python developer')
else:
  print('Some other developer')

The or is another conditional operator in python. Check this article on short circuit techniques

Logical Operators

Apart from and, or, there are few are logical operators in Python such as not, >, <, ==, >=, <=, !=

print(10 > 100) # False
print(10 < 100) # True
print(10 == 10) # True
print(10 != 50) # True
print(2 > 1 and 2 > 0) # True
print(not(True)) # False
print(not False) # True

Python operators

Some quirky operations

print(True == True) #True
print('' == 1) # False
print([] == 1) # False
print(10 == 10.0) # True
print([] == []) # True

== checks for the values on both sides. It converts the type if they are not similar.

Python has a strict checking operator is which checks for the value as well as their memory locations. It is almost similar to the === operator in JavaScript.

print(True is True) # True
print('' is 1) # False
print([] is 1) # False
print(10 is 10.0) # False
print([] is []) # False

For Loops

Loops allow to run a block of code multiple number of times. In Python, the basic form of loop is a for loop which can loop over an iterable.

for item in 'Python': # String is iterable
  print(item) # prints all characters of the string

for item in [1,2,3,4,5]: # List is iterable
    print(item) # prints all numbers one at a time

for item in {1,2,3,4,5}: # Set is iterable
    print(item)

for item in (1,2,3,4,5): # Tuple is iterable
    print(item)

Iterable

An iterable is a collection of data that can be iterated. It means the items of the collection can be processed one by one. Lists, strings, tuples, sets and dictionaries are all iterables. The action that is performed on an iterable is iteration and the current item that is being processed is called iterator.

player = {
  'firstname': 'Virat',
  'lastname': 'Kohli',
  'role': 'captain'
}

for item in player: # iterates over the keys of player
  print(item) # prints all keys

for item in player.keys(): 
  print(item) # prints all keys

for item in player.values():
  print(item) # prints all values

for item in player.items():
  print(item) # prints key and value as tuple

for key, value in player.items():
  print(key, value) # prints key and value using unpacking

range

range is an iterable object in Python used to generate a range of numbers. It is commonly used in loops and to generate a list. Range accepts 3 input parameters start, stop and step over with 2nd and 3rd optional

for item in range(10):
  print('python') # prints python 10 times

for item in range(0,10,1):
    print('hello') # prints hello 10 times

for item in range(0, 10, 2):
    print('hii') # prints hii 5 times 

for item in range(10, 0, -1):
    print(item) # prints in reverse order

print(list(range(10))) # generates a list of 10 items

enumerate

enumerate is useful when we need the index of the iterator while looping.

for key, value in enumerate(range(10)): # using unpacking techique 
  print(f'key is {key} and value is {value}') # prints key and value at the same time

That's all for today. Will cover up the remaining parts of loops and functions to finish off with the Python fundamentals.

Another day spent well 😄 Another small step taken towards my goal of learning Python.

Hope you are enjoying this series and extracting some value out of it. I have already received lots of love from the community. Thank for the support. It means a lot.

Have a good one!

Top comments (7)

pantogil • Jul 7 '20

Very good, your explanation, I clarified doubts that I had with the (blucles) and (range), I did not know (enumerate), it can be very useful in a game that I am trying to do in Python for my children to learn and master the language . Greetings, Happy Day.