CIS 192: Lecture 9 Working with APIs

Transcription

1 CIS 192: Lecture 9 Working with APIs Lili Dworkin University of Pennsylvania

2 Last Week s Quiz Consider the code l = (i for i in range(10)) What happens if I type l[0]? >>> l = (i for i in range(10)) >>> type(l) <type 'generator'> >>> l[0] TypeError: 'generator' object has no attribute ' getitem '

3 Last Week s Quiz Consider the code l = (i for i in range(10)) What about l.next()? >>> l.next() 0

4 Last Week s Quiz Consider the code l = (i for i in range(10)) What about sum(l)? >>> sum(l) 45

5 Last Week s Quiz Consider the code l = (i for i in range(10)) What about len(l)? >>> len(l) TypeError: object of type 'generator' has no len() How can we get the length?

6 Last Week s Quiz >>> l = (i for i in range(10)) >>> sum(1 for _ in l) 10

7 Last Week s Quiz Write code to return a list containing all pairs of items from the list [ a, b, c, d ]. >>> l = ['a', 'b', 'c', 'd'] >>> import itertools >>> i = itertools.combinations(l, 2) >>> i <itertools.combinations object at 0x107dacf18> >>> [j for j in i] [('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]

8 Last Week s Quiz Write a regular expression to match a date of the form MM-DD-YYYY or MM-DD-YY. >>> r''\d{2}-\d{2}-(\d{2} \d{4})'' NOT >>> r''\d{2}-\d{2}-\d{2 4}''

9 Last Week s Quiz If I call a python program with python test.py --name Lili -n 10 and parse the command-line arguments with options, args = optparser.parse args(), what will the list args contain?

10 What is HTTP? The protocol that allows for sending resources back and forth on the web. A protocol is a set of rules that determines which messages can be exchanged, and which messages are appropriate replies. Two roles: server and client This week: we are the client. Next week: we are the server! (This slide and those following courtesy of this nettuts article.)

11 HTTP Messages Made of a header and body. Body = data that you want to transmit over the network Header = instructions for how to use the data

12 URLs Send a request for a resource to a host URLs define resources (e.g. web page) Resource: /clients/jim Host: example.com Resources are nouns

13 Methods Methods are verbs/actions Tells the server what to do with the resource GET = retrieve data POST = create/update data DELETE = delete data

14 Requests/Responses Client and server exchange information about resources identified by URLs Request and response contain a representation of the resource The body can contain data in many formats: plain text, HTML, JSON...

15 JSON What s happening here? >>> req = requests.get(" >>> print req.text { "origin": " ", "url": " "args": {}, "headers": {... } } >>> req.text['origin'] TypeError Looks like a dictionary is being returned, but it s actually unicode.

16 JSON But the string has a special format, known as JSON, which is used for passing data back and forth over the internet. >>> d = req.json() >>> type(d) <type 'dict'> >>> d['origin'] u' ' >>> d['url'] u'

17 JSON JSON is built on two structures: Object: A collection of name/value pairs (i.e. dict) Array: Ordered list of values (i.e. list) A value in the above definitions can be a string (in double quotes), a number, true, false, null, or another object or array.

18 JSON Python comes with a JSON encoder and decoder: >>> import json >>> l = ['hi', {'key1': 0, 'key2': ['a', 'b']}] >>> x = json.dumps(l) >>> x '["hi", {"key2": ["a", "b"], "key1": 0}]' >>> type(x) <type 'str'>

19 JSON >>> y = json.loads(x) >>> y [u'hi', {u'key2': [u'a', u'b'], u'key1': 0}] >>> type(y) <type 'list'>

20 Content Type The response should specify the content type of the body Done in the header, in the Content-Type field: >>> req = requests.get(' >>> req.headers['content-type'] 'text/html' >>> req = requests.get(" >>> req.headers['content-type'] 'application/json'

21 Headers We can send headers in our requests, too. Remember: headers provide instructions about the transaction Defined as a dictionary: >>> headers = {} >>> req = requests.get(url, headers=headers)

22 Sending Data We can send data along with our requests For GET requests, this can be done directly in the URL, or in the params dictionary: >>> req = requests.get('.../test?name1=value1& name2=value2') OR >>> params = {'name1':'value1', 'name2':'value2'} >>> req = requests.get('.../test', params=params)

23 POST Requests What if we want to send a lot of data, and/or it s sensitive and shouldn t be in the URL? requests.post() issues a POST request POST requests submit data to be processed The data is sent in the body of the request, not in the url >>> data = {'name1': 'value1', 'name2': 'value2'} >>> req = requests.post(url, data=data) Let s practice!

24 API In the context of web development, an API (application programming interface) is a set of HTTP request messages, along with the definition of the structure of response messages, which is usually XML or JSON. APIs allow the sharing of content and data between web communities and applications.

25 Authentication Multiple ways: Single key/token, sent in a GET request Application-only authentication / 2-legged OAuth sent on behalf of an application 3-legged OAuth sent on behalf of a specific user

26 Single Key/Token Sign up to get a key and/or token Send in the parameters of a GET request to get resources Example: Google Places

27 Application-only Authentication 1. Sign up to get a consumer key and secret 2. Send your key and secret in the headers of a POST request to get an access token 3. Server grants access token in body of response 4. Send in your access token in the parameters of a GET request to get resources Example: Twitter

28 Twitter

29 Properties Remember from way back when... class Foo(object): def init (self, i): self._eep = def data(self): print "eep is being accessed." return self._eep

30 Properties >>> f = Foo(5) >>> f.data eep is being accessed. 5 >>> f._eep # don't do this 5

31 Properties Use case: lazy-load data upon def data(self): if not self._data: # execute code to get data self._data = stuff return self._data

32 Function Types >>> def hi():... return 'hi'... >>> type(hi)?

33 Function Types >>> def hi():... return 'hi'... >>> type(hi) <type 'function'> >>> isinstance(hi, function) NameError: name 'function' is not defined

34 Function Types Builtin types that don t have constructors (e.g. functions, generators, methods) are in the types module. You can use types.functiontype in an isinstance call. >>> import types >>> isinstance(hi, types.functiontype) True >>> isinstance((i for i in range(5)), types.generatortype) True

35 Function Types >>> class A(object):... def f(self, x):... return x... >>> A.f <unbound method A.f> >>> a = A() >>> a.f <bound method A.f of < main.a object at 0x10a87ac90>>

36 Function Types Bound method: a function with the first argument pre-filled as the instance (self) Unbound method: a function with the restriction that the first argument (self) must be an instance of the object >>> a.f(1) 1 >>> A.f() TypeError: unbound method f() must be called with A instance as first argument (got nothing instead) >>> A.f(a, 1) 1

37 Function Types >>> type(a.f) <type 'instancemethod'> >>> >>> type(a.f) <type 'instancemethod'> >>> isinstance(a.f, types.methodtype) True >>> isinstance(a.f, types.methodtype) True In Python 3, A.f will have type function instead!

38 Quiz 1. req = requests.get(" 1.1 type(req.text) 1.2 type(req.json) 1.3 type(req.json()) type(json.dumps(json.loads('{"key": "val"}')) 2.2 type(json.loads(json.dumps(0))) 3. Return the content type of a response object called req. 4. What is the main difference between GET and POST requests? 5. l = [lambda x: x + i for i in range(5)] 5.1 type(l) 5.2 l[0](0)