DS4UX (Spring 2016)/Day 3 lecture: Difference between revisions

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[[File:Highfivekitten.jpeg|200px|thumb|In which you learn how to use Python and web APIs to meet the likes of her!]]
== Review of some important Week 2 concepts ==
===Lists===


== Lecture Outline ==
* Use lists to store data where order matters.
;Introduction and context
* Lists are indexed starting with 0.


* You can write some tools in Python now. Congratulations!
====List initialization====
* Today we'll learn how to find/create data sets
* Next week we'll get into data science (asking and answering questions)


>>> my_list = []
>>> my_list
[]
>>> your_list = ["a", "b", "c", 1, 2, 3]
>>> your_list
['a', 'b', 'c', 1, 2, 3]


;Outline:
====Access and adding elements to a list====


* What is an API?
>>> len(my_list)
* How do we use one to fetch interesting datasets?
0
* How do we write programs that use the internet?
>>> my_list[0]
* How can we use the placekitten API to fetch kitten pictures?
Traceback (most recent call last):
* Introduction to structured data (JSON)
  File "<stdin>", line 1, in <module>
* How do we use APIs in general?
IndexError: list index out of range
>>> my_list.append("Alice")
>>> my_list
['Alice']
>>> len(my_list)
1
>>> my_list[0]
'Alice'
>>> my_list.insert(0, "Amy")
>>> my_list
['Amy', 'Alice']


>>> my_list = ['Amy', 'Alice']
>>> 'Amy' in my_list
True
>>> 'Bob' in my_list
False


;What is a (web) API?
====Changing elements in a list====


* API: a structured way for programs to talk to each other (aka an interface for programs)
>>> your_list = []
* Web APIs: like a website your programs can visit (you:a website::your program:a web API)
>>> your_list.append("apples")
>>> your_list[0]
'apples'
>>> your_list[0] = "bananas"
>>> your_list
['bananas']


====Slicing lists====


; How do we use an API to fetch datasets?
>>> her_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
>>> her_list[0]
'a'
>>> her_list[0:3]
['a', 'b', 'c']
>>> her_list[:3]
['a', 'b', 'c']
>>> her_list[-1]
'h'
>>> her_list[5:]
['f', 'g', 'h']
>>> her_list[:]
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']


Basic idea: your program sends a request, the API sends data back
==== sorting lists ====
* Where do you direct your request? The site's API endpoint.
** For example: Wikipedia's web API endpoint is http://en.wikipedia.org/w/api.php
* How do I write my request? Put together a URL; it will be different for different web APIs.
** Check the documentation, look for code samples
* How do you send a request?
** Python has modules you can use, like <code>requests</code> (they make HTTP requests)
* What do you get back?
** Structured data (usually in the JSON format)
* How do you understand (i.e. parse) the data?
** There's a module for that!


Use <code>.sort()</code> to sort a list:


; How do we write Python programs that make web requests?
>>> names = ["Eliza", "Joe", "Henry", "Harriet", "Wanda", "Pat"]
>>> names.sort()
>>> names
['Eliza', 'Harriet', 'Henry', 'Joe', 'Pat', 'Wanda']
>>> names.sort(reverse=True)
['Wanda', 'Pat', 'Joe', 'Henry', 'Harriet', 'Eliza']


To use APIs to build a dataset we will need:
==== Getting the maximum and minimum values from a list ====
* all our tools from last session: variables, etc
* the ability to open urls on the web
* the ability to create custom URLS
* the ability to save to files
* the ability to understand (i.e., parse) JSON data that APIs usually give us


>>> numbers = [0, 3, 10, -1]
>>> max(numbers)
10
>>> min(numbers)
-1


; New programming concepts:


* interpolate variables into a string using % and %()s
* requests
* open files and write to them


== New concepts for Week 3 exercises and challenges ==


; How do we use an API to fetch kitten pictures?
===More string functions===


[http://placekitten.com/ placekitten.com]
==== Formatting strings ====
* API that takes specially crafted URLs and gives appropriately sized picture of kittens
Formatting strings makes it much easier to combine alphanumeric characters and other types of object (like ints, floats, and bools) and do things with them—like print!
* Exploring placekitten in a browser:
** visit the API documentation
** kittens of different sizes
** kittens in greyscale or color
* Now we write a small program to grab an arbitrary square from placekitten by asking for the size on standard in: [http://mako.cc/teaching/2014/cdsw-autumn/placekitten_raw_input.py placekitten_raw_input.py]


>>> x = 1
>>> y = 1.234
>>> z = True
>>> w = "elevator"
>>> all_together_now = "You can put ints like %d, floating point numbers like %f, boolean values like %s, and other strings like %s into a string without changing them to strings first!" % (x,y,z,w)
>>> print(all_together_now)


; Introduction to structured data (JSON, JavaScriptObjectNotation)
==== Dealing with whitespace ====
>>> text = " this is    a text  string  with lots    of extra      spaces    "
>>> text.strip()
"this is    a text  string  with lots    of extra      spaces"
>>> text.split()
['this', 'is', 'a', 'text', 'string', 'with', 'lots', 'of', 'extra', 'spaces']
>>> " ".join(text.split())
'this is a text string with lots of extra spaces'


* what is json: useful for more structured data
* import json; json.loads()
* like Python (except no single quotes)
* simple lists, dictionaries
* can reflect more complicated data structures
* Example file at http://mako.cc/cdsw.json
* You can parse data directly with <code>.json()</code> on a <code>requests</code> call


; Using other APIs
==== Tuples ====
Tuples are similar to lists, but unlike lists, once they're created ("assigned") they can't be changed. Since most of our work involves reading and writing files and building and manipulating sets of data, we might not have too much cause to use tuples. But Python uses them a lot "behind the scenes", and they're useful for other types of programming, so we'll go over them briefly here.


* every API is different, so read the documentation!
You can create a tuple just like a list...
* If the documentation isn't helpful, search online
>>> my_tuple = ("John", "Terry", "Terry", "Graham", "Eric")
* for popular APIs, there are python modules that help you make requests and parse json


Possible issues:
You can find items by index...
* rate limiting
>>> my_tuple[1]
* authentication
'Terry'
* text encoding issues


== Other Potentially Resources ==
BUT you can't edit them...
>>> my_tuple[1] = "John"
---------------------------------------------------------------------------
TypeError                                Traceback (most recent call last)
<ipython-input-63-2dfac7e646ea> in <module>()
----> 1 my_tuple[1] = "Michael"


My friend Frances gave a version of this lecture last year and create slides. They are written for Python 2, so the code might not all work (remember, use <Code>print()</code> with parentheses) but the basic ideas might be helpful:
TypeError: 'tuple' object does not support item assignment


* [http://mako.cc/teaching/2014/cdsw-autumn/lecture2-web_apis.pdf Slides (PDF)] — For viewing
* [http://mako.cc/teaching/2014/cdsw-autumn/lecture2-web_apis.odp Slides (ODP Libreoffice Slides Format)] — For editing and modification


[[Category:DS4UX (Spring 2016)]]
====Generating a list of numbers easily with <code>range()</code>====
 
<pre>
>>> range(5)
[0, 1, 2, 3, 4]
>>> for i in range(5):
...    print("Hi" * i)
...
 
Hi
HiHi
HiHiHi
HiHiHiHi</pre>
 
The <code>range()</code> function returns a list of numbers. This is handy for when you want to generate a list of numbers on the fly instead of creating the list yourself.
 
>>> range(5)
[0, 1, 2, 3, 4]
 
Use <code>range</code> when you want to loop over a bunch of numbers in a list, or perform an operation a certain number of times:
 
>>> numbers = range(5)
>>> for number in numbers:
...    print(number * number)
...
0
1
4
9
16
 
We could rewrite the above example like this:
 
>>> for number in range(5):
...    print(number * number)
...
0
1
4
9
16
 
You can also set the start, end, and increment value (called "step") for a range.
>>> for i in range(2,20,2):
...        print(i)
2
4
6
8
10
12
14
16
18
 
=== Using break statements to halt execution ===
word_list = ["the", "quick", "brown", "fox", "jumped", "over", "the", "lazy", "dog"]
letter = "z"
seen_letter =  False
for word in word_list:
    if letter in word:
        seen_letter = True
        print("%s contains the letter %s" % (word, letter))
    else:
          print("no %s in %s" % (letter, word))
 
=== Get user input with <code>input()</code> ===
 
>>> for i in range(100):
...    my_input = input("Please type something> ")
...    if my_input == "Quit":
...        print("Goodbye!")
...        break
...    else:
...        print("You said: " + my_input)
...
Please type something> Hello
You said: Hello
Please type something> How are you?
You said: How are you?
Please type something> Quit
Goodbye!
>>>
 
=== Iterating an indeterminate number of times with <code>while</code> loops ===
 
grocery_list = []
testAnswer = input('Press y if you want to enter more groceries: ')
while testAnswer == 'y':
    food = input('Next item:')
    grocery_list.append(food)
    testAnswer = input('Press y if you want to enter more groceries: ')
 
print('Your grocery list:')
for food in grocery_list:
    print(food)
 
===Dictionaries===
 
* Use dictionaries to store key/value pairs.
* Dictionaries do not guarantee ordering.
* A given key can only have one value, but multiple keys can have the same value.
 
====Initialization====
 
>>> my_dict = {}
>>> my_dict
{}
>>> your_dict = {"Alice" : "chocolate", "Bob" : "strawberry", "Cara" : "mint chip"}
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}
 
====Types====
 
>>> type(my_dict)
<type 'dict'>
 
====Adding and removing elements ====
 
>>> your_dict["Dora"] = "vanilla"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'chocolate'}
 
>>> del your_dict["Dora"]
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}
 
====Accessing elements of a dictionary====
 
>>> your_dict["Alice"]
'chocolate'
>>> your_dict.get("Alice")
'chocolate'
 
>>> your_dict["Eve"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Eve'
>>> "Eve" in your_dict
False
>>> "Alice" in your_dict
True
>>> your_dict.get("Eve")
>>> person = your_dict.get("Eve")
>>> print(person)
None
>>> print(type(person))
<type 'NoneType'>
>>> your_dict.get("Alice")
'chocolate'
 
==== Dictionary keys can be integers, and their values can be any data type ====
 
>>> mixed_dict = {1:3, 2:'two', 3:False, 'four':['john','terry','graham']}
>>> print(mixed_dict[1])
3
>>> print(mixed_dict[2])
two
>>> print(mixed_dict[3])
False
>>> print(mixed_dict['four'][2])
graham
 
====Changing elements of a dictionary====
 
>>> your_dict["Alice"] = "coconut"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'coconut'}
 
====Looping through a dictionary====
The builtin functions <code>.items(), .keys(),</code> and <code>.values()</code> provide you with a lot of flexibility when iterating through dictionaries.
 
>>>for i in your_dict.items():
>>>    print(i)
('Bob', 'strawberry')
('Cara', 'mint chip')
('Dora', 'vanilla')
('Alice', 'chocolate')
 
>>>for i_key in your_dict.keys():
>>>    print(i_key + " is a key in this dictionary")
Bob is a key in this dictionary
Cara is a key in this dictionary
Dora is a key in this dictionary
Alice is a key in this dictionary
 
>>>for i_val in your_dict.values():
>>>    print(i_val + " is a value in this dictionary")
strawberry is a value in this dictionary
mint chip is a value in this dictionary
vanilla is a value in this dictionary
chocolate is a value in this dictionary
 
>>> for i_key, i_val in your_dict.items():
>>>    print(i_key + " is the key for " + i_val)
>>>    print(i_val + " is the value for " + i_key)
>>>    print("\n")
Bob is the key for strawberry
strawberry is the value for Bob
...
Cara is the key for mint chip
mint chip is the value for Cara
...
Dora is the key for vanilla
vanilla is the value for Dora
...
Alice is the key for chocolate
chocolate is the value for Alice
 
==== Sorting dictionaries with <code>operator</code> and <code>itemgetter</code> ====
 
We've already learned how you can use <code>.sorted()</code> to create a sorted version of a list. <code>.sorted()</code> accepts an optional <code>key</code> argument to tell it what to sort on. You can use <code>.sorted()</code> with <code>.items()</code> builtin dictionary function and the <code>itemgetter</code> function of the <code>operator</code> module to create sorted versions of dictionaries!
 
>>> import operator
>>> family = {'ozy':2, 'jonathan':34, 'portia':10, 'eva':6, 'dana':28}
>>> sorted(family.items(), key=operator.itemgetter(1), reverse=True)
[('jonathan', 34), ('dana', 28), ('portia', 10), ('eva', 6), ('ozy', 2)]
 
You can also use this approach to sort other complex data structures:
 
>>> family = [['ozy',2], ['portia',10], ['jonathan',34], ['dana', 28], ['eva', 6]]
>>> sorted(family, key=operator.itemgetter(1))
[['ozy', 2], ['eva', 6], ['portia', 10], ['dana', 28], ['jonathan', 34]]
>>> sorted(family, key=operator.itemgetter(0), reverse=True)
[['portia', 10], ['ozy', 2], ['jonathan', 34], ['eva', 6], ['dana', 28]]
 
== Exercise ==
<big>'''[http://jtmorgan.net/ds4ux/week3/notifications.zip Click here to download the scripts for this week's in-class exercise]'''</big>

Latest revision as of 04:23, 12 April 2016

Review of some important Week 2 concepts[edit]

Lists[edit]

  • Use lists to store data where order matters.
  • Lists are indexed starting with 0.

List initialization[edit]

>>> my_list = []
>>> my_list
[]
>>> your_list = ["a", "b", "c", 1, 2, 3]
>>> your_list
['a', 'b', 'c', 1, 2, 3]

Access and adding elements to a list[edit]

>>> len(my_list)
0
>>> my_list[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>> my_list.append("Alice")
>>> my_list
['Alice']
>>> len(my_list)
1
>>> my_list[0]
'Alice'
>>> my_list.insert(0, "Amy")
>>> my_list
['Amy', 'Alice']
>>> my_list = ['Amy', 'Alice']
>>> 'Amy' in my_list
True
>>> 'Bob' in my_list
False

Changing elements in a list[edit]

>>> your_list = []
>>> your_list.append("apples")
>>> your_list[0]
'apples'
>>> your_list[0] = "bananas"
>>> your_list
['bananas']

Slicing lists[edit]

>>> her_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
>>> her_list[0]
'a'
>>> her_list[0:3]
['a', 'b', 'c']
>>> her_list[:3]
['a', 'b', 'c']
>>> her_list[-1]
'h'
>>> her_list[5:]
['f', 'g', 'h']
>>> her_list[:]
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']

sorting lists[edit]

Use .sort() to sort a list:

>>> names = ["Eliza", "Joe", "Henry", "Harriet", "Wanda", "Pat"]
>>> names.sort()
>>> names
['Eliza', 'Harriet', 'Henry', 'Joe', 'Pat', 'Wanda']
>>> names.sort(reverse=True)
['Wanda', 'Pat', 'Joe', 'Henry', 'Harriet', 'Eliza']

Getting the maximum and minimum values from a list[edit]

>>> numbers = [0, 3, 10, -1]
>>> max(numbers)
10
>>> min(numbers)
-1


New concepts for Week 3 exercises and challenges[edit]

More string functions[edit]

Formatting strings[edit]

Formatting strings makes it much easier to combine alphanumeric characters and other types of object (like ints, floats, and bools) and do things with them—like print!

>>> x = 1
>>> y = 1.234
>>> z = True
>>> w = "elevator"
>>> all_together_now = "You can put ints like %d, floating point numbers like %f, boolean values like %s, and other strings like %s into a string without changing them to strings first!" % (x,y,z,w)
>>> print(all_together_now)

Dealing with whitespace[edit]

>>> text = " this is    a text  string   with lots    of extra      spaces     "
>>> text.strip()
"this is    a text  string   with lots    of extra      spaces"
>>> text.split()
['this', 'is', 'a', 'text', 'string', 'with', 'lots', 'of', 'extra', 'spaces']
>>> " ".join(text.split())
'this is a text string with lots of extra spaces'


Tuples[edit]

Tuples are similar to lists, but unlike lists, once they're created ("assigned") they can't be changed. Since most of our work involves reading and writing files and building and manipulating sets of data, we might not have too much cause to use tuples. But Python uses them a lot "behind the scenes", and they're useful for other types of programming, so we'll go over them briefly here.

You can create a tuple just like a list...

>>> my_tuple = ("John", "Terry", "Terry", "Graham", "Eric")

You can find items by index...

>>> my_tuple[1]
'Terry'

BUT you can't edit them...

>>> my_tuple[1] = "John"
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-63-2dfac7e646ea> in <module>()
----> 1 my_tuple[1] = "Michael"
TypeError: 'tuple' object does not support item assignment


Generating a list of numbers easily with range()[edit]

>>> range(5)
[0, 1, 2, 3, 4]
>>> for i in range(5):
...     print("Hi" * i)
...

Hi
HiHi
HiHiHi
HiHiHiHi

The range() function returns a list of numbers. This is handy for when you want to generate a list of numbers on the fly instead of creating the list yourself.

>>> range(5)
[0, 1, 2, 3, 4]

Use range when you want to loop over a bunch of numbers in a list, or perform an operation a certain number of times:

>>> numbers = range(5)
>>> for number in numbers:
...     print(number * number)
...
0
1
4
9
16

We could rewrite the above example like this:

>>> for number in range(5):
...     print(number * number)
...
0
1
4
9
16

You can also set the start, end, and increment value (called "step") for a range.

>>> for i in range(2,20,2):
...         print(i)
2 
4
6
8
10
12
14
16
18

Using break statements to halt execution[edit]

word_list = ["the", "quick", "brown", "fox", "jumped", "over", "the", "lazy", "dog"]
letter = "z"
seen_letter =  False
for word in word_list:
    if letter in word:
        seen_letter = True
        print("%s contains the letter %s" % (word, letter))
    else:
         print("no %s in %s" % (letter, word))

Get user input with input()[edit]

>>> for i in range(100):
...     my_input = input("Please type something> ")
...     if my_input == "Quit":
...         print("Goodbye!")
...         break
...     else:
...         print("You said: " + my_input)
... 
Please type something> Hello
You said: Hello
Please type something> How are you?
You said: How are you?
Please type something> Quit
Goodbye!
>>>

Iterating an indeterminate number of times with while loops[edit]

grocery_list = []
testAnswer = input('Press y if you want to enter more groceries: ')
while testAnswer == 'y':
    food = input('Next item:')
    grocery_list.append(food)
    testAnswer = input('Press y if you want to enter more groceries: ')
print('Your grocery list:')
for food in grocery_list:
    print(food)

Dictionaries[edit]

  • Use dictionaries to store key/value pairs.
  • Dictionaries do not guarantee ordering.
  • A given key can only have one value, but multiple keys can have the same value.

Initialization[edit]

>>> my_dict = {}
>>> my_dict
{}
>>> your_dict = {"Alice" : "chocolate", "Bob" : "strawberry", "Cara" : "mint chip"}
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}

Types[edit]

>>> type(my_dict)
<type 'dict'>

Adding and removing elements[edit]

>>> your_dict["Dora"] = "vanilla"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'chocolate'}
>>> del your_dict["Dora"]
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}

Accessing elements of a dictionary[edit]

>>> your_dict["Alice"]
'chocolate'
>>> your_dict.get("Alice")
'chocolate'
>>> your_dict["Eve"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Eve'
>>> "Eve" in your_dict
False
>>> "Alice" in your_dict
True
>>> your_dict.get("Eve")
>>> person = your_dict.get("Eve")
>>> print(person)
None
>>> print(type(person))
<type 'NoneType'>
>>> your_dict.get("Alice")
'chocolate'

Dictionary keys can be integers, and their values can be any data type[edit]

>>> mixed_dict = {1:3, 2:'two', 3:False, 'four':['john','terry','graham']}
>>> print(mixed_dict[1])
3
>>> print(mixed_dict[2])
two
>>> print(mixed_dict[3])
False
>>> print(mixed_dict['four'][2])
graham

Changing elements of a dictionary[edit]

>>> your_dict["Alice"] = "coconut"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'coconut'}

Looping through a dictionary[edit]

The builtin functions .items(), .keys(), and .values() provide you with a lot of flexibility when iterating through dictionaries.

>>>for i in your_dict.items():
>>>    print(i)
('Bob', 'strawberry')
('Cara', 'mint chip')
('Dora', 'vanilla')
('Alice', 'chocolate')
>>>for i_key in your_dict.keys():
>>>    print(i_key + " is a key in this dictionary")
Bob is a key in this dictionary
Cara is a key in this dictionary
Dora is a key in this dictionary
Alice is a key in this dictionary
>>>for i_val in your_dict.values():
>>>    print(i_val + " is a value in this dictionary")
strawberry is a value in this dictionary
mint chip is a value in this dictionary
vanilla is a value in this dictionary
chocolate is a value in this dictionary
>>> for i_key, i_val in your_dict.items():
>>>     print(i_key + " is the key for " + i_val)
>>>     print(i_val + " is the value for " + i_key)
>>>     print("\n")
Bob is the key for strawberry
strawberry is the value for Bob
...
Cara is the key for mint chip
mint chip is the value for Cara
...
Dora is the key for vanilla
vanilla is the value for Dora
...
Alice is the key for chocolate
chocolate is the value for Alice

Sorting dictionaries with operator and itemgetter[edit]

We've already learned how you can use .sorted() to create a sorted version of a list. .sorted() accepts an optional key argument to tell it what to sort on. You can use .sorted() with .items() builtin dictionary function and the itemgetter function of the operator module to create sorted versions of dictionaries!

>>> import operator
>>> family = {'ozy':2, 'jonathan':34, 'portia':10, 'eva':6, 'dana':28}
>>> sorted(family.items(), key=operator.itemgetter(1), reverse=True)
[('jonathan', 34), ('dana', 28), ('portia', 10), ('eva', 6), ('ozy', 2)]

You can also use this approach to sort other complex data structures:

>>> family = [['ozy',2], ['portia',10], ['jonathan',34], ['dana', 28], ['eva', 6]]
>>> sorted(family, key=operator.itemgetter(1))
[['ozy', 2], ['eva', 6], ['portia', 10], ['dana', 28], ['jonathan', 34]]
>>> sorted(family, key=operator.itemgetter(0), reverse=True)
[['portia', 10], ['ozy', 2], ['jonathan', 34], ['eva', 6], ['dana', 28]]

Exercise[edit]

Click here to download the scripts for this week's in-class exercise