Chapter 3: Conditionals & Exceptions¶

Boolean Expressions¶

In [1]:

42 == 42

Out[1]:

True

In [2]:

42 == 123

Out[2]:

False

In [3]:

42 == 42.0

Out[3]:

True

In [4]:

42 == 42.000000000000001

Out[4]:

True

The `bool` Type¶

In [5]:

True

Out[5]:

True

In [7]:

type(True)

Out[7]:

bool

In [8]:

False

Out[8]:

False

In [10]:

type(False)

Out[10]:

bool

In [11]:

None

In [13]:

type(None)

Out[13]:

NoneType

Singletons¶

In [16]:

[True, False, None, True, False, None]

Out[16]:

[True, False, None, True, False, None]

Relational Operators¶

In [17]:

42 == 123

Out[17]:

False

In [18]:

42 != 123

Out[18]:

True

In [19]:

42 < 123

Out[19]:

True

In [20]:

42 <= 123

Out[20]:

True

In [21]:

42 > 123

Out[21]:

False

In [22]:

42 >= 123

Out[22]:

False

Logical Operators¶

In [23]:

a = 42
b = 87

In [24]:

a > 5 and b <= 100

Out[24]:

True

In [25]:

(a > 5) and (b <= 100)

Out[25]:

True

In [26]:

a <= 5 or not b > 100

Out[26]:

True

In [27]:

(a <= 5) or not (b > 100)

Out[27]:

True

In [29]:

5 < a and a < 87

Out[29]:

True

In [30]:

5 < a < 87

Out[30]:

True

Truthy vs. Falsy¶

In [31]:

(a - 40) and (b < 100)

Out[31]:

True

In [32]:

bool(a - 40)

Out[32]:

True

In [33]:

bool(a - 42)

Out[33]:

False

In [34]:

bool(a - 44)

Out[34]:

True

In [35]:

bool(None)

Out[35]:

False

In [36]:

bool([])

Out[36]:

False

In [37]:

bool([False])

Out[37]:

True

In [38]:

bool("")

Out[38]:

False

In [39]:

bool("Lorem ipsum dolor sit amet.")

Out[39]:

True

Short-Circuiting¶

In [41]:

0 or 1

Out[41]:

In [43]:

1 or 2

Out[43]:

In [45]:

0 or 1 or 2

Out[45]:

In [47]:

False or [] or 0

Out[47]:

In [49]:

0 and 1

Out[49]:

In [51]:

1 and 0

Out[51]:

In [53]:

1 and 0 and 2

Out[53]:

In [55]:

1 and 2 and 3

Out[55]:

The `if` Statement¶

In [57]:

numbers = [7, 11, 8, 5, 3, 12, 2, 6, 9, 10, 1, 4]

In [58]:

import random

"Wrong Logic" Example: Is the number divisible by `2`, `3`, both, or none?¶

In [60]:

number = random.choice(numbers)

if number % 2 == 0:
    print(number, "is divisible by 2 only")
elif number % 3 == 0:
    print(number, "is divisible by 3 only")
elif number % 2 == 0 and number % 3 == 0:
    print(number, "is divisible by 2 and 3")
else:
    print(number, "is divisible by neither 2 nor 3")

6 is divisible by 2 only

"Correct Logic" Example: Is the number divisible by `2`, `3`, both, or none?¶

In [62]:

number = random.choice(numbers)

if number % 3 == 0 and number % 2 == 0:
    print(number, "is divisible by 2 and 3")
elif number % 3 == 0:
    print(number, "is divisible by 3 only")
elif number % 2 == 0:
    print(number, "is divisible by 2 only")
else:
    print(number, "is divisible by neither 2 nor 3")

6 is divisible by 2 and 3

"Concise Logic" Example: Is the number divisible by `2`, `3`, both, or none?¶

In [64]:

number = random.choice(numbers)

if number % 6 == 0:
    print(number, "is divisible by 2 and 3")
elif number % 3 == 0:
    print(number, "is divisible by 3 only")
elif number % 2 == 0:
    print(number, "is divisible by 2 only")
else:
    print(number, "is divisible by neither 2 nor 3")

6 is divisible by 2 and 3

Only the `if`-clause is mandatory¶

In [65]:

if random.random() > 0.5:
    print("You read this as often as you see heads when tossing a coin")

You read this as often as you see heads when tossing a coin

Common Use Case: A binary Choice¶

In [67]:

number = random.choice(numbers)

if number % 2 == 0:
    print(number, "is even")
else:
    print(number, "is odd")

6 is even

In [69]:

number = random.choice(numbers)

if number % 2:  # Note the opposite meaning!
    print(number, "is odd")
else:
    print(number, "is even")

6 is even

"Hard to read" Example: Nesting `if` Statements¶

In [71]:

number = random.choice(numbers)

# Coin is heads.
if random.random() > 0.5:
    if number % 2 == 0:
        print(number, "can be divided by 2 without a rest")
    else:
        print(number, "divided by 2 results in a non-zero rest")
# Coin is tails.
else:
    if number % 2 == 0:
        print(number, "is even")
    else:
        print(number, "is odd")

6 can be divided by 2 without a rest

"Easy to read" Example: Flattening nested `if` Statements¶

In [73]:

number = random.choice(numbers)

coin_is_heads = random.random() > 0.5
number_is_even = number % 2 == 0

if coin_is_heads and number_is_even:
    print(number, "can be divided by 2 without a rest")
elif coin_is_heads and not number_is_even:
    print(number, "divided by 2 results in a non-zero rest")
elif not coin_is_heads and number_is_even:
    print(number, "is even")
else:
    print(number, "is odd")

6 can be divided by 2 without a rest

The `if` Expression¶

$ y = f(x) = \begin{cases} 0, \text{ if } x \le 0 \\ x, \text{ otherwise} \end{cases} $

In [74]:

x = 3

if x <= 0:
    y = 0
else:
    y = x

y

Out[74]:

In [75]:

x = 3

y = 0 if x <= 0 else x

y

Out[75]:

In [76]:

x = 3

y = max(0, x)

y

Out[76]:

The `try` Statement¶

In [78]:

user_input = random.choice([0, 1, 2, 3, 4, 5])

1 / user_input

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In[78], line 3
      1 user_input = random.choice([0, 1, 2, 3, 4, 5])
----> 3 1 / user_input

ZeroDivisionError: division by zero

In [80]:

user_input = random.choice([0, 1, 2, 3, 4, 5])

try:
    print("The result is", 1 / user_input)
except:
    print("Something went wrong")

Something went wrong

In [82]:

user_input = random.choice([0, 1, 2, 3, 4, 5])

try:
    print("The result is", 1 / user_input)
except ZeroDivisionError:
    print("Something went wrong")

Something went wrong

In [84]:

user_input = random.choice([0, 1, 2, 3, 4, 5])

try:
    result = 1 / user_input
except ZeroDivisionError:
    print("Oops. Division by 0. How does that work?")
else:
    print("The result is", result)
finally:
    print("I am always printed")

Oops. Division by 0. How does that work?
I am always printed

Chapter 3: Conditionals & Exceptions¶

Boolean Expressions¶

The bool Type¶

Singletons¶

Relational Operators¶

Logical Operators¶

Truthy vs. Falsy¶

Short-Circuiting¶

The if Statement¶

"Wrong Logic" Example: Is the number divisible by 2, 3, both, or none?¶

"Correct Logic" Example: Is the number divisible by 2, 3, both, or none?¶

"Concise Logic" Example: Is the number divisible by 2, 3, both, or none?¶

Only the if-clause is mandatory¶