Lecture 5

Python Basics - if-else Chains; Slicing; Functions; while & for loops; List & Dictionary Methods; try-except Blocks

Byeong-Hak Choe
bchoe@geneseo.edu

SUNY Geneseo

February 3, 2025

Booleans, Conditions, and if Statements

Booleans, Conditions, and if Statements

10 == 20
10 == '10'
  • Boolean data have either True or False value.

Booleans, Conditions, and if Statements

  • Existing booleans can be combined, which create a boolean when executed.

Booleans, Conditions, and if Statements

Conditions are expressions that evaluate as booleans.

Booleans, Conditions, and if Statements

boolean_condition1 = 10 == 20
print(boolean_condition1)

boolean_condition2 = 10 == '10'
print(boolean_condition2)

  • The == is an operator that compares the objects on either side and returns True if they have the same values

  • Q. What does not (not True) evaluate to?

  • Q. Classwork 4.2

Booleans, Conditions, and if Statements

name = "Geneseo"
score = 99

if name == "Geneseo" and score > 90:
    print("Geneseo, you achieved a high score.")

if name == "Geneseo" or score > 90:
    print("You could be called Geneseo or have a high score")

if name != "Geneseo" and score > 90:
    print("You are not called Geneseo and you have a high score")
  • The real power of conditions comes when we start to use them in more complex examples, such as if statements.

Booleans, Conditions, and if Statements

name_list = ["Lovelace", "Smith", "Hopper", "Babbage"]

print("Lovelace" in name_list)

print("Bob" in name_list)
  • One of the most useful conditional keywords is in.
    • This one must pop up ten times a day in most coders’ lives because it can pick out a variable or make sure something is where it’s supposed to be.
  • Q. Check if “a” is in the string “Sun Devil Arena” using in. Is “a” in “Anyone”?

Booleans, Conditions, and if Statements

score = 98

if score == 100:
    print("Top marks!")
elif score > 90 and score < 100:
    print("High score!")
elif score > 10 and score <= 90:
    pass
else:
    print("Better luck next time.")
  • One conditional construct we’re bound to use at some point, is the if-else chain:

Indentation

  • We have seen that certain parts of the code examples are indented.

  • Code that is part of a function, a conditional clause, or loop is indented.

  • Indention is actually what tells the Python interpreter that some code is to be executed as part of, say, a loop and not to executed after the loop is finished.

Indentation

x = 10

if x > 2:
    print("x is greater than 2")

  • Here’s a basic example of indentation as part of an if statement.

  • The standard practice for indentation is that each sub-statement should be indented by 4 spaces.

Slicing Methods with Strings and Lists

Slicing Methods

  • With slicing methods, we can get subset of the data object.

  • Slicing methods can apply for strings, lists, arrays, and DataFrames.

  • The above example describes indexing in Python

Strings

string = "cheesecake"
print( string[-4:] )
  • From strings, we can access the individual characters via slicing and indexing.
string = "cheesecake"
print("String has length:")
print( len(string) )
list_of_numbers = range(1, 20)
print("List of numbers has length:")
print( len(list_of_numbers) )
  • Both lists and strings will allow us to use the len() command to get their length:

Strings and Slicing

  • We can extract a substring (a part of a string) from a string by using a slice.

  • We define a slice by using square brackets ([]), a start index, an end index, and an optional step count between them.

    • We can omit some of these.

  • The slice will include characters from index start to one before end:

Get a Substring with a Slice

letters = 'abcdefghij'
letters[:]
  • [:] extracts the entire sequence from start to end.
letters = 'abcdefghij'
letters[4:]
letters[2:]
letters[-3:]
letters[-50:]
  • [ start :] specifies from the start index to the end.
letters = 'abcdefghij'
letters[:3]
letters[:-3]
letters[:70]
  • [: end ] specifies from the beginning to the end index minus 1.
letters = 'abcdefghij'
letters[2:5]
letters[-26:-24]
letters[35:37]
  • [ start : end ] indicates from the start index to the end index minus 1.
letters = 'abcdefghij'
letters[2 : 6 : 2]   # From index 2 to 5, by steps of 2 characters
letters[ : : 3]     # From the start to the end, in steps of 3 characters
letters[ 6 : : 4 ]    # From index 19 to the end, by 4
letters[ : 7 : 5 ]    # From the start to index 6 by 5:
letters[-1 : : -1 ]   # Starts at the end and ends at the start
letters[: : -1 ]
  • [ start : end : step ] extracts from the start index to the end index minus 1, skipping characters by step.

Lists

  • Python is
    • a zero-indexed language (things start counting from zero);
    • left inclusive;
    • right exclusive when we are specifying a range of values.

Lists

list_example = ['one', 'two', 'three']
list_example[ 0 : 1 ]
list_example[ 1 : 3 ]

  • We can think of items in a list-like object as being fenced in.
    • The index represents the fence post.

Lists

Get an Item by [index]

suny = ['Geneseo', 'Brockport', 'Oswego', 'Binghamton', 
        'Stony Brook', 'New Paltz']
  • We can extract a single value from a list by specifying its index:
suny[0]
suny[1]
suny[2]
suny[7]
suny[-1]
suny[-2]
suny[-3]
suny[-7]
  • We can extract a subsequence of a list by using a slice:
suny = ['Geneseo', 'Brockport', 'Oswego', 'Binghamton', 
        'Stony Brook', 'New Paltz'] 
suny[0:2]    # A slice of a list is also a list.
suny[ : : 2]
suny[ : : -2]
suny[ : : -1]
suny[4 : ]
suny[-6 : ]
suny[-6 : -2]
suny[-6 : -4]

Slicing Methods

Functions, Arguments, and Parameters

Functions

int("20") 
float("14.3")
str(5)
int("xyz")

  • A function can take any number and type of input parameters and return any number and type of output results.

  • Python ships with more than 65 built-in functions.

  • Python also allows a user to define a new function.

  • We will mostly use built-in functions.

Functions, Arguments, and Parameters

print("Cherry", "Strawberry", "Key Lime")
print("Cherry", "Strawberry", "Key Lime", sep = "!")
print("Cherry", "Strawberry", "Key Lime", sep=" ")

  • We invoke a function by entering its name and a pair of opening and closing parentheses.

  • Much as a cooking recipe can accept ingredients, a function invocation can accept inputs called arguments.

  • We pass arguments sequentially inside the parentheses (, separated by commas).

  • A parameter is a name given to an expected function argument.

  • A default argument is a fallback value that Python passes to a parameter if the function invocation does not explicitly provide one.

Functions, Arguments, and Parameters

Loop with while and for

Repeat with while

count = 1        
while count <= 5:
    print(count)
    count += 1
  1. We first assigned the value 1 to count.
  2. The while loop compared the value of count to 5 and continued if count was less than or equal to 5.
  3. Inside the loop, we printed the value of count and then incremented its value by one with the statement count += 1.
  4. Python goes back to the top of the loop, and again compares count with 5.
  5. The value of count is now 2, so the contents of the while loop are again executed, and count is incremented to 3.
  6. This continues until count is incremented from 5 to 6 at the bottom of the loop.
  7. On the next trip to the top, count <= 5 is now False, and the while loop ends.

Asking the user for input

stuff = input()
# Type something and press Return/Enter on Console 
# before running print(stuff)
print(stuff)
  • Sometimes we would like to take the value for a variable from the user via their keyboard.
    • The input() function gets input from the keyboard.
    • When the input() is called, the program stops and waits for the user to type something on Console (interactive Python interpreter).
    • When the user presses Return or Enter on Console, the program resumes and input returns what the user typed as a string.

Cancel with break

while True:
    user_input = input("Enter 'yes' to continue or 'no' to stop: ")
    if user_input.lower() == 'no':
        print("Exiting the loop. Goodbye!")
        break
    elif user_input.lower() == 'yes':
        print("You chose to continue.")
    else:
        print("Invalid input, please enter 'yes' or 'no'.")
  • While loop is used to execute a block of code repeatedly until given boolean condition evaluated to False.
    • while True loop will run forever unless we write it with a break statement.
  • If we want to loop until something occurs, but we’re not sure when that might happen, we can use an infinite loop with a break statement.

Repeat with while

Skip Ahead with continue

while True:
    value = input("Integer, please [q to quit]: ")
    if value == 'q': # quit
        break
    number = int(value)
    if number % 2 == 0: # an even number
        continue
    print(number, "squared is", number*number)

  • Sometimes, we don’t want to break out of a loop but just want to skip ahead to the next iteration for some reason.

  • The continue statement is used to skip the rest of the code inside a loop for the current iteration only.

Repeat with while

Check break Use with else

  • We can consider using while with else when we’ve coded a while loop to check for something, and breaking as soon as it’s found. 
numbers = [1, 3, 5]
position = 0

while position < len(numbers):
    number = numbers[position]
    if number > 4:  # Condition changed to checking if the number is greater than 4
        print('Found a number greater than 4:', number)
        break
    position += 1
else:  # break not called
    print('No number greater than 4 found')
  • Consider it a break checker.

Iterate with for and in

  • Sometimes we want to loop through a set of things such as a string of text, a list of words or a list of numbers.

    • When we have a list of things to loop through, we can construct a for loop.

    • A for loop makes it possible for you to traverse data structures without knowing how large they are or how they are implemented.

Iterate with for and in

  • Let’s see two ways to walk through a string here:
word = 'thud'
offset = 0
while offset < len(word):
    print(word[offset])
    offset += 1
word = 'thud'
for letter in word:
    print(letter)
  • Which one do you prefer?

Iterate with for and in

Cancel with break

word = 'thud'
for letter in word:
    if letter == 'u':
        break
    print(letter)
  • A break in a for loop breaks out of the loop, as it does for a while loop:

Iterate with for and in

Skip with continue

word = 'thud'
for letter in word:
    if letter == 'u':
        continue
    print(letter)
  • Inserting a continue in a for loop jumps to the next iteration of the loop, as it does for a while loop.

Generate Number Sequences with range()

  • The range() function returns a stream of numbers within a specified range, without first having to create and store a large data structure such as a list or tuple.

    • This lets us create huge ranges without using all the memory in our computers and crashing our program.

    • range() returns an iterable object, so we need to step through the values with for … in, or convert the object to a sequence like a list.

for … in range()

for x in range(0, 3):
    print(x)
list( range(0, 3) )
  • We use range() similar to how we use slices: range( start, stop, step ).
    • If we omit start, the range begins at 0.
    • The only required value is stop; as with slices, the last value created will be just before stop.
    • The default value of step is 1, but we can change it.

Iterate with for and in

Check break Use with else

  • Similar to while, for has an optional else that checks whether the for completed normally.
    • If break was not called, the else statement is run.
word = 'thud'
for letter in word:
    if letter == 'x':
        print("Eek! An 'x'!")
        break
    print(letter)
else:
    print("No 'x' in there.")

Loop with while and for

Class Exercises

List and Dictionary Comprehensions

List Comprehension

What is List Comprehension?

  • A concise way to create or modify lists.
  • Syntax: [expression for item in iterable if condition]
  1. Creating a List of Squares:
squares = [x**2 for x in range(5)]
  1. Filtering Items:
numbers = [1, 2, 3, 4, 5, 6]
evens = [x for x in numbers if x != 2]

Dictionary Comprehension

What is Dictionary Comprehension?

  • A concise way to create or modify dictionaries.
  • Syntax: {key_expression: value_expression for item in iterable if condition}
  1. Creating a Dictionary of Squares:
squares_dict = {x: x**2 for x in range(5)}
  1. Filtering Dictionary Items:
   my_dict = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
   filtered_dict = {k: v for k, v in my_dict.items() if v != 2}
  1. Swapping Keys and Values:
original_dict = {'a': 1, 'b': 2, 'c': 3}
swapped_dict = {v: k for k, v in original_dict.items()}

Modifying Lists and Dictionaries

Modifying a List

Adding Items

  • append(): Adds an item to the end of the list.
my_list = [1, 2, 3]
my_list.append(4)

Modifying a List

Deleting Items

  • remove(): Deletes the first occurrence of value in the list.
my_list = [1, 2, 3, 4, 2]
my_list.remove(2)
  • List Comprehension: Removes items based on a condition.
my_list = [1, 2, 3, 4, 2]
my_list = [x for x in my_list if x != 2]
  • del statement: Deletes an item by index or a slice of items.
my_list = [1, 2, 3, 4]
del my_list[1] 
del my_list[1:3]

Modifying a Dictionary

Adding/Updating Items

  • update(): Adds new key-value pairs or updates existing ones.
my_dict = {'a': 1, 'b': 2}
my_dict.update({'c': 3})  
my_dict.update({'a': 10})

Modifying a Dictionary

Deleting Items

  • Dictionary Comprehension: Removes items based on a condition.
my_dict = {'a': 1, 'b': 2, 'c': 3}
my_dict = {k: v for k, v in my_dict.items() if v != 2}
  • del statement: Deletes an item by key.
my_dict = {'a': 1, 'b': 2, 'c': 3}
del my_dict['b']

Handle Errors with try and except

Handle Errors with try and except

Exception handlers

Handle Errors with try and except

Exception handlers

  • In some languages, errors are indicated by special function return values.

    • Python uses exceptions: code that is executed when an associated error occurs.

  • When we run code that might fail under some circumstances, we also need appropriate exception handlers to intercept any potential errors.

    • Accessing a list or tuple with an out-of-range position, or a dictionary with a nonexistent key.

Handle Errors with try and except

Errors

short_list = [1, 2, 3]
position = 5
short_list[position]
  • If we don’t provide your own exception handler, Python prints an error message and some information about where the error occurred and then terminates the program:

Handle Errors with try and except

short_list = [1, 2, 3]
position = 5

try:
    short_list[position]
except:
    print('Need a position between 0 and', len(short_list)-1, ' but got',
    position)
  • Rather than leaving things to chance, use try to wrap your code, and except to provide the error handling:

Handle Errors with try and except

short_list = [1, 2, 3]
position = 5
try:
    short_list[position]
except:
    print('Need a position between 0 and', len(short_list)-1, ' but got',
    position)
  • The code inside the try block is run.
    • If there is an error, an exception is raised and the code inside the except block runs.
  • If there are no errors, the except block is skipped.

Handle Errors with try and except

except type

  • Specifying a plain except with no arguments, as we did here, is a catchall for any exception type.

  • If more than one type of exception could occur, it’s best to provide a separate exception handler for each.

  • We get the full exception object in the variable name if we use the form:

Handle Errors with try and except

except type

short_list = [1, 2, 3]
while True:
    value = input('Position [q to quit]? ')
    if value == 'q':
        break
    try:
        position = int(value)
        print(short_list[position])
    except IndexError as err:
        print('Bad index:', position, '-', err)
    except Exception as other:
        print('Something else broke:', other)

Handle Errors with try and except

except type

  • The example looks for an IndexError first, because that’s the exception type raised when we provide an illegal position to a sequence.

  • It saves an IndexError exception in the variable err, and any other exception in the variable other.

  • The example prints everything stored in other to show what you get in that object.

    • Inputting position 3 raised an IndexError as expected.
    • Entering two annoyed the int() function, which we handled in our second, catchall except code.

Handle Errors with try and except

Class Exercises

Importing and Installing Modules, Packages, and Libraries

Importing Modules, Packages, and Libraries

  • Python is a general-purpose programming language and is not specialized for numerical or statistical computation.

  • The core libraries that enable Python to store and analyze data efficiently are:

    • pandas
    • numpy
    • matplotlib and seaborn

Importing Modules, Packages, and Libraries

pandas

  • pandas provides Series and DataFrames which are used to store data in an easy-to-use format.

Importing Modules, Packages, and Libraries

numpy

  • numpy, numerical Python, provides the array block (np.array()) for doing fast and efficient computations;

Importing Modules, Packages, and Libraries

matplotlib and seaborn

  • matplotlib provides graphics. The most important submodule would be matplotlib.pyplot.
  • seaborn provides a general improvement in the default appearance of matplotlib-produced plots.

Importing Modules, Packages, and Libraries

import statement

  • A module is basically a bunch of related codes saved in a file with the extension .py.

  • A package is basically a directory of a collection of modules.

  • A library is a collection of packages

  • We refer to code of other module/package/library by using the Python import statement.

import LIBRARY_NAME
  • This makes the code and variables in the imported module available to our programming codes.

Importing Modules, Packages, and Libraries

import statement

as

  • We can use the as keyword when importing the module/package/library using its canonical names.
import LIBRARY as SOMETHING_SHORT

from

  • We can use the from keyword when specifying Python module/package/library from which we want to import something.
from LIBRARY import FUNCTION, PACKAGE, MODULE

Installing Modules, Packages, and Libraries

pip tool

  • To install a library LIBRARY on your Google Colab, run:
!pip install LIBRARY
  • To install a library LIBRARY on your Anaconda Python, open your Spyder IDE, Anaconda Prompt, or Terminal and run:
pip install LIBRARY