notes.md

Basic Python Notes

1. Hello world program
2. Program flow
   • Program draw tree
   • Python runs line by line:
   • expression - a piece of code that produces a value
   • strings
3. Variable
   • variable - temporarily store data in computer's memory.
   • when python executes $a = 5$ expression, allocates memory, stores the data into the memory, attaches the label to that memory (identifier)
   • types: integer number, floating point number, boolean values, string
   • Exercise: define and print variable for name, age, boolean flag.
4. Input from user
   • What is your name program
   • build-in functions
   • Exercise : favorite color,
5. Type conversion,
   • Program computes age
   • errors
   • int(), float(), bool()
   • type()
   • Exercise: input kg, output pounds
6. Strings:
   • ' inside string use ", " inside string use '
   • ''' for multiline strings
   • +operator, *operator, int
   • indexing [], negative index!!
   • slicing with :, e.g. [0:3], excludes last, start index and end index if missed
   • Question: what is the result of [1:-1]
   • formatted string, generates strings, better than + for complex expression, works with placeholders, e.g. f'{var1} text {var2}'
   • function len - general purpose
   • . operator, methods difference function <> method
   • Exercise: find method upper, lower, title
   • Question: does upper change the value of the variable (intellisense helps)
   • method find,search for occurrence of string
   • Question: what does it return on failure, is it case sensitive
   • method replace
   • in operator, very similar to find
   • Question: what is the type of the result of in operator?
7. Arithmetic operations and math functions
   • int and float types
   • +/-/* operators
   • several type of division operations: / floating point result, // whole number, % modulus
   • power operator **
   • augmented assignments
   • Exercise: compare result of augmented assignments
   • Question: 10 + 3 * 2 ** 2. What is the precedence of the operators above?
   • built-in round and abs
   • What is a module? A separate file with reusable code. Modules are used to organize the code into files.
   • methods in the module are accessed with the . operator
   • Question: math.ceil; math.floor; math.fabs vs abs
   • Exercise: google "python 3 math module", spend 5 min browsing through the functions
8. if statement
   • code block refers to a collection of code that is in the same block or indent. This is most commonly found in classes, functions, and loops.
   • Exercise: if is_hot output that "It's a hot day". On new line: "Drink plenty of water." Else if its is_cold output "It's a called day. Wear warm clothes." Otherwise output "It's a lovely day." And finally "Enjoy your day."
   • logical and operator : combining two conditions, both have to be true, example is_sunny and is_average_temperature
   • logical or, either has to be true, example is_sunny or is_average_temperature
   • logical not, inverts the value, example is_sunny and not is_windy
   • comparison operators: boolean expression that compares values
   • Question: Guess all comparison operators? (>,<,>=,<=,==,!=)
   • Exercise: same as above but with temperature
   • Exercise: input name. if len < 3 output invalid name, too short, if len > 30 print invalid name, too long, otherwise valid
   • Exercise: Enter weight. Answer Question (L)bs or (K)gs? and convert to the other, should not be case sensitive.
9. While loop
   • Loops are used to execute a block of code multiple times
   • In the while loop the block is executed as long as the condition is True
   • Example: print i from 1 to 5
   • Question: what will happen if we don't increase i?
   • Exercise: Step through the loop with the debugger.
   • Exercise: Draw half of a pine tree.
   • Break and Continue statements: skip steps with continue, break loop
   • Loops can have else
   • Exercise: Build a guessing game. (use random.randrange(1, 10))
   • Exercise: Build a car game (commands: start, stop, help, quit)
10. For loop
    • More convenient to iterate over items in a collection
    • Loop variable: holds item for the current iteration
    • Example: for character in 'Python'...
    • Example: for name in ['Name 1' , 'Name 2', ...]
    • Example: for num in [1, 2, 3, ...]
    • Range function: creating range of numbers. It's not a list, but it's an iteratable object. Takes also beginning of the range, and step
    • Exercise: Calculate the sum of the elements of a list
    • Exercise: Nested loops, chess board coordinates
    • Exercise: Step through the nested loops
    • Exercise: Write the letter F with numbers
11. Lists
    • Exercise: define a list of names and print it in the terminal
    • Individual elements can be accessed with indexing (remember negative indices?)
    • Slicing, selecting a range
    • Exercise: Find the largest number in a list
    • 2 dimensional lists: define a matrix
    • Exercise: use nested loops to iterate over the matrix and print each element
    • Question: remember the difference between function and method?
    • append method, adds an item to the end
    • insert method, adds an item to specified position
    • remove method, removes first occurrence
    • clear method, removes all items
    • pop method, removes last item
    • index method, checks existence of item, value error if not found
    • in operator, similar but boolean result
    • count method, counts occurrences
    • sort, reverse methods
    • Question: Assign the list to another variable and modify an element. Does it affect the list? Now try copy method.
    • Exercise: Collect duplicates from one list in another list
12. Tuple
    • Similar to list, storing a list of items. Defined with '()' unlike list '[]'
    • But tuples are immutable.
    • Question: Define a tuple. Check the methods of a tuple. Do we have append, insert
    • Question: What methods are there (similar to lists)? Do you remember what can we do with count and index
    • Question: can we index a tuple with []?
    • Question: can we modified a value from the tuple?
    • This is useful for defining constants.
    • Unpacking of tuple: x,y,z = coordinates
    • Question: Can you unpack a list?
13. Set and Dictionary
    • Set is a collection of unique values. Defined with '{}' rather than '[]' or '()'.
    • Question: What is the type of a variable defined with empty '{}'? For empty sets call set().
    • Question: Define a set with three equal values. Can you unpack it to three different variables?
    • Exercise: Similar to above. Collect all unique elements from a list to into a set.
    • Similar to set. Dictionary stores information as pairs of (unique!) key and value.
    • Dictionary is a collection of data, useful because can represent real world entities.
    • Question: Can we have keys of different types?
    • Access with '[]', throws a key error if no such key.
    • '[]' can be used to define a new key value pair however.
    • get method doesn't throw, returns a None object or default value.
    • Exercise: Convert a number to words
    • Exercise: Emoji converter (use split method, converts string to list, '\U0001F642' smiling face, '\U0001F641')
14. Functions
    • We need functions to: reuse code and for readability (to break the code into manageable chunks)
    • A function is a way to contain code that performs a specific task
    • Question: which built-in functions do you remember? What is the purpose of each of these functions?
    • Example: function for saying hallo
    • def key word, followed by function name and () and :. Always use well chosen descriptive names
    • Best practices for formatting python code: Two lines after function end.
    • Question: Do the expression in the function body get executed if the function is not called?
    • Exercise: Use the debugger
    • Question: What will happen if we call the function before it's defined.
    • Question: What is the difference between the defined function and built-in print
    • Function parameters are used to pass information to the function. They are placeholder for receiving information.
    • Parameters act like local variables.
    • Exercise: call the defined function with two different parameters.
    • Question: What will happen if you call the function without an argument?
    • Exercise: add a second parameter
    • Positional arguments: the position or order is important
    • Exercise: reverse the order and observe the result.
    • Keyword arguments: the position doesn't matter
    • Normally we use positional arguments, sometimes keyword arguments improve readability
    • Keyword arguments always come after positional arguments.
    • Functions can also return values with return statement.
    • Exercise: write a function that computes the square of a number
    • Exercise: extract the emoji converter code into a function
15. Exceptions
    • Question: Write a program that asks for the age of the user and converts it to int. What is the exit code of the program? What if we enter invalid number?
    • We should anticipate errors and handle them, don't let the program crash
    • Use try-except construct to handle errors
    • In the try block we write the code that we expect to not be safe
    • In the except block we write the code that deals with a specific error
    • We can have many except blocks for each error type
    • Exercise: compute chance to go to Mars by a formula (1/age)
    • If no error in except is specified, we catch all errors.
    • Rasing errors with raise
    • Exercise: write chance-to-visit function and raise ValueError if age is negative/zero
16. Modules
    • Module: A file with python code.
    • Modules are used to organize the program source code into files. Each files contains functionality that is logically grouped together
    • Also we can reuse code by importing modules in different parts of the project.
    • Exercise: write two functions for converting pounds to kg and vice versa in a module and import from another py file
    • Import specific functions from a module: form <module> import <func>
    • Exercise: create and import max_number module.
    • A package is a container for multiple modules. It's basically a folder with files (modules).
    • It contains a special python file __init__.py
    • Exercise: create and import a "greeting" module, from a "messages" package. Test importing package and specific module.
    • Google: 'python 3 module index'
    • pathlib module, Path methods: absolute(), exist(), mkdir(), rmdir(), glob() - "accepts search pattern", iteratable result
17. File I/O
    • open() function - read/write stream depending on the mode.
    • reading methods: read(), readline(), readlines()
    • writing methods: write(), writelines()
    • Files must be closed: simulate crash with os._exit(1) and see how the change is lost.
    • same example with with statement: makes sure that a file handle is closed with open('filename') as f:
18. Regular expressions
    • RegEx: Expressions for patterns in text
    • Can be used for validation and matching of certain formats and limitations
    • Same can be achieved with if-else statements and loops with a lot of verbosity
    • RegEx are shorter but often more convoluted, complicated and harder to read. Error-prone.
    • regex101.com
    • re module
    • re.compile(<pattern>) for compiling a pattern that can be used in matches and searches
    • Example: upper case word, use anchors
    • difference between match and search: match only looks for matches from the beginning of the string
    • Exercises: extend the pattern to include lower case letters. How do you include an empty space?
    • Example: "^[a-z]{3}[0-9]{3,5}[^a-zA-Z0-9]{1}[A-Z]{0,2}$"
    • Alternatives: \w, \d, \W \D \s \S, *, +, ?
    • Exercise: match an email address
    • finditer method to iterate through matches
    • (|) - group of alternatives, Example: match Mr Ms or Mrs Name
    • Capture information from groups, Example: match urls https:://www.google.com, https://youtube.com, http://zamunda.se
    • group method of match
    • findall returns a list of matches for each group, tuple for many groups
19. Classes
    • Classes are used to define new types. Data + methods for handle the data
    • class keyword, e.g. class Point:
    • def keyword for defining methods, e.g. def move(self):
    • self reference to a particular instance inside the methods.
    • Example: define a new Point type with 'move' and 'draw' methods, prints to simulate action
    • Instance: an object of a given type, e.g. point1 = Point()
    • Question: what are the methods avaiable for the defined object?
    • Atrributes: act like variable belonging to a particular object.
    • Add attributes the objects by simply initializing them, e.g. point.x = 4
    • Exercise: print the attributes of the object, print the attribute of another object.
    • Constructor: a function that is called when creating a object
    • def __init__ to define a custom constructor, e. g. def __init__(self, x, y)
    • Inside ctor, we are initializing attributes to the current object using self
    • Exercise: Define type Person with name attribute and method talk.
    • We can inherit classes, parent class put in '()'
    • Methods and attributes of the parent are passed to the new type
    • Example: class Mammal, class Dog, class Cat, share walk method
    • pass statement for derived classes without extra definitions.