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Lecture 2

Overview

Developing programs in Python.

Functions in Python

  • print() prints the arguments to the screen.

  • input("Prompt:")is used to get input from the user. No matter what you type, this function will return a string. So, you must cast the return value to whatever type you need it to be.

  • if-elif-else is equivalent to if, else if, and else in other languages like Java and C.

  • int() takes anything and turns it into an integer.

  • To define your own function, use def. For example:

    def square(x):
    return x * x
for i in range(10):
    print(f"The square of {i} is {square(i)}")

  • __init__ creates a class object by storing inputs, if any, inside of the object.

  • In a class definition, the word "self" is used in the same way as the "this" key word in Java. It is used to referer to the class object itself.

Terminologies and things to know

  • Python is an interpreted language, meaning if you run a program called python, which is an interpreter, it will read your py file line by line, executing each line and interpreting what it is that it means in a way that the computer can understand.

  • To run a py file, type in your terminal python <filename.py>

  • Python doesn't require you to explicitly state what the type of each variable is.

  • Boolean values can be represented using True with a capital T or False with a capital F.

  • NoneType: A special type in python which has only one possible value, a capital N, None. It is used to represent the lack of a value somewhere. If a function is not returning anything, it is returning None effectively.

  • '+' operater will add 2 numbers or concatenate 2 strings.

  • fstrings are formatted strings. put an f before quotation marks. For example:

name = input("Name:")
print(f"Hello, {name}!")
  • lambda is a condensed verion of a function. For example:
def f(person):
    return person['name']

people.sort(key = f)

# is the same as

people.sort(key = lambda person : person['name'])

  • Indentation is required because the indentation is how the program knows what code is inside of the if statement and what code is outside of the if statement.

  • Sequences in Python are data types that store values in some sort of sequence or some collection of values altogether. Multiple elements can have the same value.

  • Mutable means you can change the elements, i.e., add new elements, remove elements, and change the values of elements.

  • Immutable mean you cannot change the elements.

  • Types of Sequences:

    1. String. Can access different characters within the string using string[i] where string is the string whose character you want to extract and i is the index of the required character. Methods:
      • len(string)gives the number of characters in a string.
    2. List: Can store any type of data inside a list in Python. It is a sequence of mutable values.
    3. Tuple: A tuple is used when you have a couple of values that are not going to change but you need to store 2 or more values together. Sequence of immutable values.

  • Types of Data Structures:

    1. List: Methods:
      • .append(i) to add a new element 'i' to the end of the list.
      • .sort() to sort the elements in a particular order. The order is not user defined.
      • len(list) gives the number of elements inside a list.
    2. Tuple
    3. Set: collection of unique values. Does not keep things in a particular order. Methods:
      • .add(i) to add a new element 'i', not in a particular order.
      • .remove(i) to remove a particular element 'i' from anywhere in the set.
      • len(set) gives the number of elements inside a set.
    4. dict: collection of key-value pairs.
    # define a dictionary
ages = {"Alice": 22, "Bob": 27}

# accessing a particular value using a key
print(ages["Alice"])

# to add a new key-value pair to a dict
ages["Charlie"] = 30

  • To write programs that interact with csv files, we need to import the csv module.

Loops in Python

  • For Loop: 
    for i in [0, 1, 2, 3, 4]:
    print(i)

# is equivalent to 

for i in range(5):
    print(i)
  • For each loop 
    for element in list:
    print(element)

# or

for character in string:
    print(character)

Object Oriented Programming

  • In OOP we think about the worl in terms on objects where objects might store information inside of them and also support the ability to perform some operations or methods or functions on them.

  • A class is a template for a type of object. You can create your own types by defining your own classes.

Functional Programming

  • Decorators are a way in Python of taking a function, and modifying that function, adding some additional behavior to that function.

  • A decorator takes a function as input and returns a modified version of that function as output.

  • To add a decorator use the "@" symbol.

Exceptions

  • try and except similar to Java's try-catch block.

  • if you import the pre-defined sys module, you can say sys.exit(1) to mean exit the program with a status code of 1 generally means something went wrong in this program.