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Functions and (list and string methods)
Python SIG – PYA Class 3 – 3/10/15
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.)
(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
(Revision of)if else elif
if SomethingThatEvaluatesToABoolean:# code
elif SomethingElseThatEvaluatesToABoolean:# code
else:# code
(Revision of)String formatting
• What is it? (Immutable)• %something - %d, %f, %s – ‘a = %d’ % (a)
• Prefer .format()• {} sufficient – numbers optional• More complex things possible
(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
(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: 1\na\n3
And one more thingDon’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
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/2012/07/09/16-common-python-runtime-errors/
Obligatory xkcd reference[1]
Serious questions:
• What are functions?• Why are they important?• What is the difference between a
function and a method? (OOP language guys should answer)
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 yourprogram and any number of times. This is known as calling the function.” - Byte of Python, Swaroop CH[2]
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?
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)
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 itdescribes what the function does“””
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
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...
Remember import antigravity?
• For those who don’t:
(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)?
Why am I talking about that anyway?
• Because whatever we import are usually .pyor .pyc files
• And I found antigravity.py and antigravity.pyc in C:\Python27\Lib
• 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?
(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!
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!
Make a dice rolling gameFind 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.
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/
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):
*args and **kwargs
• Variable number of arguments• kwargs – keyword arguments• def complex_func(*args, **kwargs):
# try thisa = args; b = kwargsprint(a, type(a)); print(b,type(b))
*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?
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.
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
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()
AssignmentWrite 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’.
AssignmentA 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!
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()
Palindrome checker
• Write a palindrome checker.• That’s it. Up to you how complex you make it.
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
