![(Revision of)for loops can do more!
• What is this ‘in’ anyway? (different time
complexity for different cases)
• for char in string
• for line in text
• for item in sequence
–Example: for i in [1,’a’,3]:
print i
# output: 1nan3](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-7-320.jpg)
![Obligatory xkcd reference[1]](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-10-320.jpg)
![Functions are:
“Functions are reusable pieces of programs.
They allow you to give a name to a block of
statements, allowing you to run that block
using the specified name anywhere in your
program and any number of times. This is
known as calling the function.” - Byte of
Python, Swaroop CH[2]](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-12-320.jpg)
![General syntax
• def function_name(param_1, ..., param_n):
# code to do something
[return some_tuple] # square brackets because
# optional
• Why param not arg?
• That is, what is the difference between
arguments and parameters?
• Fact: Functions in Python always return
something. So, in a function that just prints
something, what does it return?](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-13-320.jpg)
![More about functions
• Flow of execution – jumping when called,
ending when a return is reached
• Try making a IndentationError or SyntaxError
in your function definition. Is it caught without
calling your function?
• What about other types of errors? That is,
runtime errors.
( dir(__builtins__) lists (yes, LISTs), among
other things, all possible [Something]Error)](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-14-320.jpg)
![Default args
• Functions can use a default value as argument
in case it is not passed while calling it.
• “Only those parameters which are at the end
of the parameter list can be given default
argument values ” [2]
• def func_name(param1, param2 = 0):
• NOT def func_name(param1 = 0, param2):](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-25-320.jpg)
![List methods
• Do dir([]) and try to guess what each of the
methods might do to a list.
• list.append(element)
• list.extend(list)
• list.remove(element)
• list.pop(index)
• list.reverse()
• list.sort()](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-30-320.jpg)
![Assignment
Write a function which takes an input ‘n’.
Then it asks for ‘n’ numbers and makes a list
of Boolean values that are returned by a
function is_greater(arg1, arg2) by comparing
input values in order. That is, when 4 values a,
b, c and d are given. The value of the list is:
[a > b, b > c, c > d]. The program runs until the
user types ‘end’.](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-31-320.jpg)
![Assignment
A list containing ints, floats, and strings, when unsorted
looks like this:
Example: l1_unsorted = [[1], 1, 2, 1.0, ‘aa’, ‘a’, ‘b’,
[‘b’], [‘a’]]
Using the default sort gives:
Example: l1_sorted = [1, 1.0, 2, [1], ['a'], ['b'], 'a', 'aa',
'b']
Write a program that returns:
l1_custom_sorted = [[‘a’], [‘b’], ‘b’, ‘aa’, ‘a’, 2, 1,
1.0] . Make sure that no more than 4 lines of code is
outside a function. Functionify you code!](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-32-320.jpg)
![String methods
• Do dir(‘’) and try to guess what each of
methods might do to a string.
• string.split(delimiter)
• string.upper() / .lower()
• ‘ ‘.join([list of things to join])
• string.startswith() / .endswith()](https://image.slidesharecdn.com/class3-151222173157/85/Functions-List-and-String-methods-33-320.jpg)
Functions, List and String methods
- 1. Functions and (list and string methods) Python SIG – PYA Class 3 – 3/10/15
- 2. These are things you should know pretty well by now: • raw_input, printing • help(), dir(), online docs, searching the internet • The fact that Python is awesome. (No, really, I’m just being completely objective here.)
- 3. (Revision of)Useful built-ins • len() • range() (and xrange()) • sum(), max(), min() • int(), float(), str() • dir() – use it to find list and string methods • Use help() to find out about them
- 4. (Revision of)if else elif if SomethingThatEvaluatesToABoolean: # code elif SomethingElseThatEvaluatesToABoolean: # code else: # code
- 5. (Revision of)String formatting • What is it? (Immutable) • %something - %d, %f, %s – ‘a = %d’ % (a) • Prefer .format() • {} sufficient – numbers optional • More complex things possible
- 6. (Revision of)Loops • for – for when you how many iterations • while - while you don’t know how many iterations • while SomethingThatEvaluatesToABoolean: # code • for loop syntax in Python – for iterVar in (x)range(iterNum): # code
- 7. (Revision of)for loops can do more! • What is this ‘in’ anyway? (different time complexity for different cases) • for char in string • for line in text • for item in sequence –Example: for i in [1,’a’,3]: print i # output: 1nan3
- 8. And one more thing Don’t be afraid of these: Exception EOFError OSError StopIteration ImportError SyntaxError SystemExit KeyboardInterrupt IndentationError StandardError LookupError SystemError ArithmeticError IndexError SystemExit OverflowError KeyError TypeError FloatingPointError NameError ValueError ZeroDivisonError UnboundLocalError RuntimeError AssertionError EnvironmentError NotImplementedError AttributeError IOError SomeRandomError
- 9. And one more thing • If nothing, look at what error it is and what line it is on. And make sure you look above and below that line. • Further reading – 16 common runtime errors Python beginners find: http://inventwithpython.com/blog/2012/07/0 9/16-common-python-runtime-errors/
- 11. Serious questions: • What are functions? • Why are they important? • What is the difference between a function and a method? (OOP language guys should answer)
- 12. Functions are: “Functions are reusable pieces of programs. They allow you to give a name to a block of statements, allowing you to run that block using the specified name anywhere in your program and any number of times. This is known as calling the function.” - Byte of Python, Swaroop CH[2]
- 13. General syntax • def function_name(param_1, ..., param_n): # code to do something [return some_tuple] # square brackets because # optional • Why param not arg? • That is, what is the difference between arguments and parameters? • Fact: Functions in Python always return something. So, in a function that just prints something, what does it return?
- 14. More about functions • Flow of execution – jumping when called, ending when a return is reached • Try making a IndentationError or SyntaxError in your function definition. Is it caught without calling your function? • What about other types of errors? That is, runtime errors. ( dir(__builtins__) lists (yes, LISTs), among other things, all possible [Something]Error)
- 15. Docstrings • Let’s open some random library’s .py files and look at the code. (Be careful about modifying it, though.) • Have you noticed __doc__? • Ignoring classes (and double underscores) for now. • def somefunction(args): “””This is a docstring and it describes what the function does “””
- 16. Functions that return • What is the point of this? • How to receive what the function returns? • Be careful about number of arguments. • Idea of local variables. • Global variables using global statement (But bad practice!) • Functional programming – stateless, no side effects
- 17. Imported stuff • Before going to assignments for functions, let’s take a little detour to the import statement. • Modules vs. Libraries – basic difference • And before going to import, let’s take a little detour to...
- 18. Remember import antigravity? • For those who don’t:
- 19. (not so) Serious questions: • Do you think I’m just looking for an excuse to put xkcd comics in my slides or they actually have served a purpose in every case (so far)?
- 20. Why am I talking about that anyway? • Because whatever we import are usually .py or .pyc files • And I found antigravity.py and antigravity.pyc in C:Python27Lib • Let’s open both with a text editor. • What happens when we click on both? • What happens when we modify the .py file? • Does is effect the .pyc file? Should it?
- 21. (Again)Why am I talking about that anyway? • To explain better how import works – The .pyc file is unaffected until we modify, save and import antigravity again – Searches everything in sys.path • Just to give you an idea that Python is not a complex or rigid as you think it is. • Open source for the win!
- 22. Try ‘this’. Literally. • Try ‘import this’. • Find “this.py” and try to make sense of it. • Make a .py file called “this_1.py” and modify it to print whatever you want. • Save it in a non C: partition • Now find a way to run it in a Python session without navigating to saved location. • After you’ve finished, read the output of import this. It’s pretty well written!
- 23. Make a dice rolling game Find a module that helps you with pseudorandom generation of numbers and use it to create a program to emulate a dice. Then it should roll the dice until the user gets the same number three times in a row or m tries, whichever is earlier. (m is entered by the user at the beginning) In case of the former, it prints, “You’re lucky.” Otherwise it prints “m chances up. ” It should also print no. of chances remaining at every roll and display the results of the last 3 rolls.
- 24. For those who have finished • Try playing the game to see if you “are lucky”! How big should your m be to win consistently? • What is the ideal probability for a random dice? (1/6th per toss, one doesn’t affect the other.) • But here it is pseudorandom. • Further reading – Blog post by Jeff Atwood: https://blog.codinghorror.com/computers- are-lousy-random-number-generators/
- 25. Default args • Functions can use a default value as argument in case it is not passed while calling it. • “Only those parameters which are at the end of the parameter list can be given default argument values ” [2] • def func_name(param1, param2 = 0): • NOT def func_name(param1 = 0, param2):
- 26. *args and **kwargs • Variable number of arguments • kwargs – keyword arguments • def complex_func(*args, **kwargs): # try this a = args; b = kwargs print(a, type(a)); print(b,type(b))
- 27. *args and **kwargs • You can use for loops to iterate over the arguments. • args is a tuple • kwargs is a dictionary • How to call such a function? What is the value of the dictionary?
- 28. Other things about functions • Functions can return multiple values. Use tuples (if confused, just comma-separate the variables for now) • Too many arguments make a function argumentative. Limit yourself to fewer arguments per function; and more functions if required.
- 29. List and string methods • Why is this important enough to be a separate topic? • What is the main difference between them? • Hint: This goes back to mutability. • Hint: Try something on both. What is the return type
- 30. List methods • Do dir([]) and try to guess what each of the methods might do to a list. • list.append(element) • list.extend(list) • list.remove(element) • list.pop(index) • list.reverse() • list.sort()
- 31. Assignment Write a function which takes an input ‘n’. Then it asks for ‘n’ numbers and makes a list of Boolean values that are returned by a function is_greater(arg1, arg2) by comparing input values in order. That is, when 4 values a, b, c and d are given. The value of the list is: [a > b, b > c, c > d]. The program runs until the user types ‘end’.
- 32. Assignment A list containing ints, floats, and strings, when unsorted looks like this: Example: l1_unsorted = [[1], 1, 2, 1.0, ‘aa’, ‘a’, ‘b’, [‘b’], [‘a’]] Using the default sort gives: Example: l1_sorted = [1, 1.0, 2, [1], ['a'], ['b'], 'a', 'aa', 'b'] Write a program that returns: l1_custom_sorted = [[‘a’], [‘b’], ‘b’, ‘aa’, ‘a’, 2, 1, 1.0] . Make sure that no more than 4 lines of code is outside a function. Functionify you code!
- 33. String methods • Do dir(‘’) and try to guess what each of methods might do to a string. • string.split(delimiter) • string.upper() / .lower() • ‘ ‘.join([list of things to join]) • string.startswith() / .endswith()
- 34. Palindrome checker • Write a palindrome checker. • That’s it. Up to you how complex you make it.
- 35. Thanks! Pranav S Bijapur, ECE, BMSCE, Bangalore [email protected] Telegram - @pranavsb
- 36. References All (Revision of) slides were taken from the class2 presentation that was made by me and used on 29/9/15 1. xkcd – Wisdom of the ancients https://xkcd.com/979 2. Byte of Python by Swaroop CH, available online at http://www.swaroopch.com/notes/python