Programming in Python

Poll: How can I swap the values of x: int and y: int?

a)

temp: int = x
x = y
y = temp

b)

temp: int = x
temp = y
y = x

c)

temp: int = y
x = y
x = temp

d)

temp: int = x
y = temp
x = y

Choose how to represent data

String manipulation

Strings in Python can be represented using single quotes ('cat') or double quotes ("cat").

One way to represent a double quote in a string is to represent the string using single quotes, and vice versa:

• 'This is a double quote: "'
• "This is a single quote: '"

We can also represent a literal double quote in the string using an escape sequence to indicate that it shouldn't end the string: "This is a double quote: \""

Here is a list of string escape sequences:

• "\t" tab character
• "\n" newline character
• "\r" carriage return
• """ double quote character
• "'" single quote character
• "\" backslash character

String concatenation

• "Joseph" + "Aoun"
• "Joseph" + " " + "Aoun"
• We can multiply a string by an integer: "!" * 10

Poll: What does this print? print("I am so excited" + "!" * 3)

1. I am so excited! I am so excited! I am so excited!
2. I am so excited!I am so excited!I am so excited!
3. I am so excited!!!​
4. Nothing – it breaks

F-string​

We can put a variable directly in a string to save time.

cats: int = 4
print(f"There are {cats} cats in this room."​)


>> There are 4 cats in this room.

It also helps reduce the number of places where a bug can happen in the code.

Float

We've seen these types in the examples so far: int, str.

What if we want to represent a number that isn't an integer? Like 2.5? Then we use a float.

num: float = 2.5

Tip: dividing any two numbers in Python results in a float -- even if the operands are ints, and the result has an integer value.

print(type(4 / 2))

>> <class 'float'>

Boolean

• Store the result of a yes-no question​
• Only 2 (legit) values: True and False
• We're assuming you've seen booleans before

my_decision: bool = True

Some boolean operators

• Opposite
  • not my_decision​
• Comparisons: <, <=, >, >=, ==, !=
  • 4 < 6
• And (both must be true to result in true)​
  • my_decision and your_decision
• Or (true if either/both are true)​
  • my_decision or your_decision

Order of operations

• Math order of operations: 2. (Parentheses) 3. Exponents
  • 4**2 * 3 >> 48
  4. Multiplication/Division (left to right)
  5. Addition/Subtraction (left to right)
• Math happens before comparison operations
  • 7 < 2 + 8 >> True
• Comparison happens before boolean operations​
  • 3 < 4 and 5 < 7 True

None

None works like a value that represents the absence of a value.

bodyguard_name: str = None # doesn't have a value -- I don't have a bodyguard

It's different from "" or 0 (see Null Island)

Can store None in a list​

grades: List[int] = [5, None, 0]

Cannot add None to a number or string

• None + "hi" does not work
• len(None) does not work​

To specify that a type might be None, we use Optional. For example:

from typing import Optional

def get_number_or_None(hopefully_a_number: str) -> Optional[int]:
    try:
        return int(hopefully_a_number)
    except ValueError:
        return None

Types

We started using types in our Python code since day one (last lecture). Here are two new types, which need to be imported:

from typing import List, Set

nums: List[int] = [1, 2, 3]
words: Set[str] = {'hi', 'hello', 'howdy'}

We're assuming you have used lists before. Sets may be new to some.

A set is very similar to a list: it is a collection of items.

Differences between a set and a list:

• A set is unordered
• A set can only hold each item (at most) once -- no duplicates​

There will be more about sets in Lecture 12.

Control structures

We're assuming you've seen conditionals and iteration before, though possibly in a different programming langauge. Here it is in Python:

Conditionals

If / else

secret_num: int = 8
guess: int = int(input('My guess: '))

if secret_num == guess:
    print('I guessed it!')
elif (secret_num + 1 == guess) or (secret_num - 1 == guess):
    print('So close!')
else:
    print('Maybe next time!')

Tip: we can put a conditional expression in one line:

print('yes' if my_decision else 'no')
print(f'{num_cats} cat{'s' if num_cats > 1 else ''}')

Match case statements

If there are many cases, a match-case statement might be more practical:

name: str = input('Please enter your name: ')
match name:
    case 'SpongeBob':
        print('You are a sponge')
    case 'Patrick':
        print('You are a starfish')
    case _:
        print('I don\'t know you')

A match-case statement finds the first case that matches​, and only executes that one case (or zero cases if none match).

The case _ is a catch-all that matches anything that didn't fit any other cases. It is not required, but if it is there, it must be the last case.

Iteration

While loops

animal: str = input('Please enter an animal: ')

while not is_animal(animal):
    animal = input('That wasn\'t an animal. Please enter an animal: ')

For loops over numbers

We use while loops when we don't know in advance how many iterations we will need. If we do know the number of iterations (given the variables we currently have), then a for loop is more appropriate.

For loops in Python can use a helpful function called range():

for i in range(4):
    print(i)

>> 0
   1
   2
   3

We can start a range at a number other than 0:

for i in range(2, 5):
    print(i)

>> 2
   3
   4

We can also ask it to count in "steps" larger than 1:

for i in range(10, 50, 5):
    print(i)

>> 10
   15
   20
   25
   30
   35
   40
   45

For loops over the elements of a collection

It turns out that the range() function returns a collection, which the for loop iterates over. We can instead tell Python to iterate over the elements of a different collection:

for character in 'I love cats!':
    print(character.upper())

>> I
 
   L
   O
   V
   E
 
   C
   A
   T
   S
   !

Poll: What's wrong with this function? Why doesn't the docstring match the code?

"""Function to generate a random float"""
from random import random

def sarcasm(phrase: str) -> str:
    """Returns the sarcastic version of the provided phrase, where a 
    randomly selected half of the characters are uppercase, and the 
    others are lowercase.
    
    Parameters
    ----------
    phrase : str
        The phrase to turn sarcastic
    
    Returns
    -------
    str
        The sarcastic version of the phrase
    """
    sarcastic_phrase = ''
    for character in phrase:
        if random() < 0.5:
            sarcastic_phrase += character.upper()
    return sarcastic_phrase

1. It's adding the index of the character, not the character itself
2. It skips adding about half of the letters
3. Sometimes, it doesn't return a string at all
4. It adds extra characters to the string

Poll: Which of these is a one-line version of the inside of the (correct) sarcasm() function?

1. return ''.join([character.upper() for character in phrase if random() < 0.5])
2. return ''.join([character.upper() if random() < 0.5 for character in phrase])
3. return ''.join([character.upper() if random() else character.lower() for character in phrase])
4. return ''.join([character.upper() if random() < 0.5 else character.lower() for character in phrase])

For loops over a collection, keeping track of indices

for index, word in enumerate(['American Shorthair', 'Balinese', 'Cheetah']):
    print(f'{index}: {word}')

>> 0: American Shorthair
   1: Balinese
   2: Cheetah

Read and write data from user input and text files

We've been using the input('prompt') function which returns the user's response to the provided 'prompt'.

We can also read data from a file:

with open('story.txt', 'r', encoding="utf-8") as file:
    for line in file.readlines():
        print(line)

If, instead of reading from the file, we want to write to the file, then we must use a different option than 'r'.

• open('story.txt', 'r'): read the file
• open('story.txt', 'w'): write the file (overwrite it if it already exists)
• open('story.txt', 'a'): append to the end of the file (and create the file if it doesn't exist)

We can then write to the file using file.write("Line to write to file").

Import code

We've been importing modules like import unittest and from typing import List, Set.

We can also import code from a file that we wrote ourselves: import my_file

When a Python file is imported, all of the code inside it is executed. (Try it out -- put print('hello') in a new file and import it.) That's why we put our code inside functions -- we don't want the code inside to be executed when it's imported!

In a function named main(), we call all the functions that we want to run when the file is run (not imported).

And we add this at the end of the file so that the main() function is only called when the file is run, not imported:

if __name__ == '__main__':
    main()

Try this out using today's lecture code -- what happens if you keep all those print() statements outside of functions, and then import the file? Does it get fixed when you move that code into functions which are only called in main()? (Don't forget the if __name__ == '__main__' conditional at the end!)