Learning to Program with Python

1. Learning to Programwith Python Advanced Class 1

2. Hello World • Figured we may as well….it’s just one line. • print(“hello world”) • print() is one of python’s 68 built-in functions.

3. Basic Types • The four basic types of python are int, float, bool, and string (strfor short). • There is no char. In python, ‘c’ is a string of length 1. • ‘hello’ is a sequence of 5 shorter strings, not 5 characters.

4. Variables • Variables in python are not statically typed. They can assume a new type at any time. • myVar = 2 • myVar = “now it’s a string” • myVar = 3.0 • You can assign any valid value to any variable at any time.

5. Math • Nothing new here. • 5 + 5 – 10 * 18 + (1 + 7) • The division operator will always return a float, even with ints. • 1 / 1  1.0

6. Math • There is an integer division operator, too. • 4 / 3  1.333333 • 4 // 3  1

7. Incrementing and such • Python does not have n++ and n--. But it does have: • n = n + 1 • n += 1 • There’s also: • -= • *= • /= • //= (and several others.)

8. The input() function • user_input = input(“Tell me your name!”) • Prompts the user in the shell, then returns what they type in as a string. • Here, we assign the return value to the newly declared variable user_input.

9. String concatenation • resultString = “one fish ” + “two fish” • The variable resultString now holds the string “one fish two fish”

10. Type conversion • “stringy stuff” + 50 • That’s an ERROR! Python can’t convert implicitly. • “stringy stuff” + str(50)  “stringy stuff50” • str() is another built-in function, for type conversion. • int(“40”)  40

11. That’s enough info to get started. • Time to write our first python script.

12. A simple ATM transaction. • Using: • strings, ints • variables • type conversion • string concatenation • print() and input() • comparisons • if/elif/else statements.

13. Booleans • True and False are reserved words in python. They are capitalized. • 4 > 5, “hi” == “dog”, 1 != 2 • These are all boolean expressions that evaluate to True or False. • my_example_var = True

14. Looping in Python • The while loop works like in every language. • Python uses indentation to determine scope. while condition: indented code to be executed as long as it’s indented, it’s part of the loop this code is now back outside the loop

15. Lists • Python’s word for a dynamic array. • Elements can be of different types. • Declared with square brackets • myList = [] #this variable now holds an empty list

16. Comments #by the way • Single-line comments are done with the hash tag. • Multi-line comments are done with triple quotes. ’’’This is a multi- -line comment’’’ • More on triple-quoted comments later– there’s more to them than meets the eye.

17. Back to Lists • Internally implemented as an array of pointers in C, to put it in straightforward terms. • Has many handy methods: append, extend, pop, count, membership testing, etc. • Subscriptable! print(myList[7])

18. For loops in Python • Works a bit different than lower-level languages. • Designed to be intuitive and human readable. myList = [“hello”, “how”, “are”, “you?”] for item in myList: print(item)

19. For loops in Python • Besides iterating over a list with a for…in… loop, you can also loop over a set of numerical values like you may be used to, using the built-in range() function. for x in range(10): print(x) • Prints 0,1,2,3,4,5,6,7,8,9

20. More built-in functions • Here are some more built-ins that happen to work very well with lists: • max(list) returns the highest value in the list. • min(list) returns the lowest value in the list.

21. More built-in functions • sorted(list) returns a sorted version of the list. • sum(list) returns the sum of all the elements. • len(list) returns the length of the list.

22. Functions in Python defname_of_function(arg1, arg2…): indented code is part of function scope • The keyword is “def”, short for “define”. • Function names, just like variable names, can have letters, numbers, and underscores, but cannot start with a number. Almost as if….functions were variables….hmm…. • Functions must be declared before they can be used.

23. Let’s write another program. • We’re going to create a function that takes a base 10 number and returns a Roman Numeral string.

24. Scope in Python • I hate interview scope puzzles. • Code should be clear enough that the reader isn’t wondering “which MyVagueVariable is he referring to now???” • But it’s important to understand anyway, and in Python it’s fairly straightforward.

25. Scope in Python withSimple Functions • Functions can see outside their own scope so long as you don’t redeclare the variable. But they cannot change an external variable. • From the main scope, you cannot see into a function’s scope.

26. Scope in simple functions x = 4 def test(): x = 2 print(x) test() • test() looks inside its own scope, finds x, and prints it.

27. Scope in simple functions x = 4 def test(): print(x) test() • test() looks in its own scope, finds nothing, so it checks the outer scope, and finds x there.

28. Scope in simple functions x = 4 def test(): print(x) x = 2 test() • ERROR! The function knows you have redeclared x. It knows that a variable `x` exists in its local scope, so it refuses to look at the external scope at all. • So it just errors because you’re trying to reference a variable without declaring it.

29. Global Variables in Python • There is a global keyword, if you’re into that sort of thing. • It’s usually prudent to avoid global variables, as you’ve no doubt heard before. • If you really must use it….Google it or ask after class.

30. So much more • I would like to tell you about mutability. • I would like to tell you about list comprehensions. • I would like to tell you about functional programming with Python. • I would like to tell you about iterables, iterators, and generators. • I would like to tell you about magic methods. • I would like to tell you about function decorators.

31. So much more • Those are all somewhat in-depth topics which will give you a very, very strong grasp on how Python works. • Until we can establish a firm, basic understanding of the language, it’s best to avoid most of that. • So let’s write more code.

32. A word about the Standard Library • Python has an extremely comprehensive standard library, including but not limited to: • Math, data structures, database access, XML, HTML, threading, randomness, regular expressions, JSON, email, cryptography, csv, os access, ZIP, tarfile, gzip, time, date, HTTP cookies, sockets, codecs, and much more

33. Helpful Links • I’m going to put helpful links in a new topic under “Discussions” on the Meetup site, including: • Documentation for built-in functions • Python execution/memory visualization tool. • Module of the Week, the best introductory guide to the standard library.

34. Future Classes • Dictionaries, object-oriented programming with classes, recursion, the os module, the sys module, regular expressions, tuples, file access, sockets, in-depth understanding with magic methods, function decorators