Basic Python Notes
Tags: python, beginners, webdev, examples
Published: 2021-12-10 21:22:45

# Basics of Python


```py

import sys
print('Hello, World')
print("It's raining!")
print('''
Welcome to Python
Have a great day!!!
''')

a = None
print(a)
```


#### Arithmetic in Python
#### - numeric types: integer and float
#### - add, subtract, multiply => notice numeric types of results
#### - powers, division
#### - integer division & modulo teaming up
#### - warning: watch for rounding errors
```py
x = 25    # integer
y = 17.0  # float

 print(x)
 print(y)

 print(x + y)
 print(x - y)

 print(x * y)
 print(x / y)

 print(x // y) # integer division
 print(x % y) # modulo

 print(f'The result is {int(x // y)} remainder {int(x % y)}')

 print(x ** 2)
 print(y ** 2)

 x = 25
 y = 17.6
```


###  rounding errors due to floats
###  we can cast to int, round(num, digits), etc.

```py

 print(x * y)
 print(int(x * y))
 print(round(x * y, 2))
```
#### Casting will truncate (floor) our float
```py
print(int(17.2))
print(int(17.9))
```
#### Simple Input and Formatted Printing
#### - Prompt for input()
#### - Formatted printing 4 ways
```py
name = input('What is your name?\n')

print('Hi, ' + name + '.')
print('Hi, %s.' % name)
print('Hi, {fname} {lname}.'.format(lname="Doe", fname="John"))
print(f'Hi, {name}.')
```
## Duck Typing
#### - len()
#### - try ... except
```py
a = False
try:
    print(len(a))
except:
    print(f'{a} has no length')
```

```py
# More complex try blocks

a = 'False'
b = 6
c = 2
try:
    print(len(a))
    print(b/c)
    print(a[47])
except TypeError:
    print(f'{a} has no length')
except ZeroDivisionError as err:
    print(f'Cannot divide by zero! Error: {err}')
except:
    print(f'Uh oh, unknown error: {sys.exc_info()}')
else:
    print('No errors! Nice!')
finally:
    print('Thanks for using the program!')
# Arithmetic with Strings

a = "a"
b = "b"
an = "an"

print(b + an)
print(b + a*7)
print(b + an*2 + a)

print("$1" + ",000"*3)

# Length of a string
print(len("Spaghetti"))    # => 9

# Indexing into strings
print("Spaghetti"[0])    # => S
print("Spaghetti"[4])    # => h
print("Spaghetti"[-1])    # => i
print("Spaghetti"[-4])    # => e

# Indexing with ranges
print("Spaghetti"[1:4])  # => pag
print("Spaghetti"[4:-1])    # => hett
print("Spaghetti"[4:4])  # => (empty string)
print("Spaghetti"[:4])  # => Spag
print("Spaghetti"[:-1])    # => Spaghett
print("Spaghetti"[1:])  # => paghetti
print("Spaghetti"[-4:])    # => etti

# Using invalid indices
# print("Spaghetti"[15])    # => IndexError: string index out of range
# print("Spaghetti"[-15])    # => IndexError: string index out of range
print("Spaghetti"[:15])    # => Spaghetti
print("Spaghetti"[15:])    # => (empty string)
print("Spaghetti"[-15:])    # => Spaghetti
print("Spaghetti"[:-15])    # => (empty string)
print("Spaghetti"[15:20])    # => (empty string)

# .index() function
# Returns the first index where the character is found
print("Spaghetti".index("t"))    # => 6
# print("Spaghetti".index("s"))    # => ValueError: substring not found

# .count() function
# Returns the number of times the substring is found
print("Spaghetti".count("h"))    # => 1
print("Spaghetti".count("t"))    # => 2
print("Spaghetti".count("s"))    # => 0

# .split() function
# Returns a list (array) of substrings, split on the character passed
# If no character is passed, a space is used to split on
print("Hello World".split())    # => ["Hello", "World"]
print("i-am-a-dog".split("-"))    # => ["i", "am", "a", "dog"]

# .join() function
# Works in reverse from what you may be used to with JavaScript
# Called on a string that should be used to join each substring from the passed list
print(" ".join(["Hello", "World"]))    # => "Hello World"
# ["Hello", "World"].join(" ") JavaScript
print("-".join(["i", "am", "a", "dog"]))    # => "i-am-a-dog"

# .upper() and .lower() transformation functions
# These functions do not mutate
a = 'Hello'
print(a)
print(a.upper())
print(a)

# Some testing methods
# islower()
# isupper()
# startswith("substring")
# endswith("substring")
# isalpha() - only letters
# isalnum() - letters and numbers
# isdecimal() - only numbers
# isspace() - only whitespace
# istitle() - only title-cased letters (does not account for special words like 'a')
print('Once Upon A Time'.istitle())  # => True
print('Once Upon a Time'.istitle())  # => False
# Assignment Operators in Python
# - Increment (no postfix/prefix)
# - Powers and Integer division
# - Big Numbers
# - Stopping a runaway process (control+c)

i = 1
# i++ does not exist in Python, we have to use i += 1
i += 1
print(i)  # > 2

i += 4
print(i)  # > 6

i **= 2
print(i)  # > 36

i //= 10
print(i)  # > 3

i *= 10**200
print(i)  # > 3 followed by 200 0s (all written out)

print(float(i))  # > 3e+200 (floats are written in scientific notation)

i = 3
i **= 10**200
print(i)  # runaway process! control+c triggers a KeyboardInterrupt to stop it

# Meaning of Truth in Python
# - numeric types equivalent, but not strings
# - conditionals (if, elif, else)
# - truth equivalence

a = 1
b = 1.0
c = "1"

# print(a == b)
# print(a == c)
# print(b == c)

# if (a == c):
#     print("match")
# elif (a == b):
#     print("a matches b")
# else:
#     print("not a match")

a = []
# Falsy Values:
# 0, 0.0, 0j (complex number)
# ''
# False
# None
# []
# ()
# {}
# set()
# range(0)

if (a):
    print(f'{a} is true')
else:
    print(f'{a} is false')

# Logical Operators
# We use the keywords 'and', 'or', and 'not' in Python instead of &&, ||, and !
print(True and False)  # > False
print(True or False)  # > True
print(True and not False)  # > True

# Grouping Conditions
# Parentheses can group our conditions, just like JavaScript
print(False and (True or True))  # > False
print((False and True) or True)  # > True

# Short Circuiting
# If we can already determine the overall outcome of a compound conditional
# Python will not bother evaluating the rest of the statement
# False and (anything else) is always False, so the print is not evaluated
False and print("not printed")
# Cannot determine overall value so we have to evaluate the right side
False or print("printed #1")
# Cannot determine overall value so we have to evaluate the right side
True and print("printed #2")
# True or (anything else) is always True, so the print is not evaluated
True or print("not printed")

# JavaScript use case of short circuiting
# const composeEnhancers = window.__REDUX_DEVTOOLS_EXTENSION_COMPOSE__ || compose;
# While loops follow a very similar structure to JavaScript
i = 0
while i < 5:
    print(f'{i+1}. Hello, world.')
    i += 1

# The 'continue' keyword goes to the next loop
# The 'break' keyword exits out of the loop completely
i = 0
while True:
    print(f'{i+1}. Hello, world.')
    if i < 4:
        i += 1
        continue
    print("You've printed 5 times. Goodbye.")
    break

# Identity vs. Equality
# - is vs. ==
# - working with literals
# - isinstance()

a = 1
b = 1.0
c = "1"

print(a == b)
print(a is b)

print(c == "1")
print(c is "1")

print(b == 1)
print(b is 1)

print(b == 1 and isinstance(b, int))
print(a == 1 and isinstance(a, int))

# d = 100000000000000000000000000000000
d = float(10)
e = float(10)

print(id(d))
print(id(e))
print(d == e)
print(d is e)

b = int(b)
print(b)
print(b == 1 and isinstance(b, int))

print(a)
print(float(a))
print(str(a))

print(str(a) is c)
print(str(a) == c)

# Make an xor function
# Truth table
# | left  | right | Result |
# |-------|-------|--------|
# | True  | True  | False  |
# | True  | False | True   |
# | False | True  | True   |
# | False | False | False  |
# def xor(left, right):
#   return left != right


def xor(left, right): return left != right


print(xor(True, True))  # > False
print(xor(True, False))  # > True
print(xor(False, True))  # > True
print(xor(False, False))  # > False


def print_powers_of(base, exp=1):
    i = 1
    while i <= exp:
        print(base ** i)
        i += 1


# We are not hoisting the function declaration, we need to invoke after declared
print_powers_of(15)
print_powers_of(exp=6, base=7)
print_powers_of(2, 5)
print_powers_of(3, 5)
print_powers_of(10, 5)

if True:
    x = 10

print(x)
print(i)


def greeting_maker(salutation):
    print(salutation)

    def greeting(name):
        return f"{salutation} {name}"
    return greeting
# print(salutation) # Error, salutation is not defined at this scope


hello = greeting_maker("Hello")
hiya = greeting_maker("Hiya")

print(hello("Monica"))
print(hello("Raja"))

print(hiya("Raul"))
print(hiya("Tariq"))
```
```

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/oxgd7r73mtx2q835j85g.jpg)