Advanced Flask

We continue using Flask in this module with a look at more complex endpoints and data retrieval functions for our REST API. After going through this module, students should be able to:

  • Identify valid and invalid Flask route return types

  • Convert unsupported types (e.g. list) to valid Flask route return types

  • Extract Content-Type and other headers from Flask route responses

  • Add query parameters to GET requests, and extract their values inside Flask routes

  • Deal with errors from user-supplied input to an API and handle Python exceptions

Note

We will continue to do our work on the isp02 VM. Like last time, it will be helpful for you to have two SSH terminals open to isp02 at the same time so you can run your Flask application in one terminal and test it in the other.

Defining the URLs of Our API

One of our first goals for our API will be to provide an interface to a dataset. Since the URLs in a REST API are defined by the “nouns” or collections of the application domain, we can use a noun that represents our data.

For example, suppose we have the following dataset that represents the number of students earning an undergraduate degree for a given year:

def get_data():
    return [ {'id': 0, 'year': 1990, 'degrees': 5818},
             {'id': 1, 'year': 1991, 'degrees': 5725},
             {'id': 2, 'year': 1992, 'degrees': 6005},
             {'id': 3, 'year': 1993, 'degrees': 6123},
             {'id': 4, 'year': 1994, 'degrees': 6096} ]

In this case, one collection described by the data is “degrees”. So, let’s define a route, /degrees, that by default returns all of the data points.

EXERCISE 1

Create a new file, degrees_api.py to hold a Flask application then do the following:

  1. Import the Flask class and instantiate a Flask application object.

  2. Add code so that the Flask server is started when this file is executed directly by the Python interpreter.

  3. Copy the get_data() method above into the application script.

  4. Add a route (/degrees) which responds to the HTTP GET request and returns the complete list of data returned by get_data() as a Python string. Hint: You won’t be able to return a Python list directly from your route function – consider casting the list to a string using the str() function.

In a separate Terminal use curl to test out your new route. Does it work as expected?

Tip

Refer back to the Intro to Flask material if you need help remembering the boiler-plate code.

Responses in Flask

If you tried to return the list object directly in your route function definition, you got an error when you tried to request it with curl. Something like:

TypeError: The function did not return a valid response

Flask allows you three options for creating responses:

  1. Return a string (str) object

  2. Return a dictionary (dict) object

  3. Return a tuple (tuple) object

  4. Return a flask.Response object

Some notes:

  • Option 1 is good for text or html such as when returning a web page or text file.

  • Option 2 is good for returning rich information in JSON-esque format.

  • Option 3 is good for returning a list of data using a special type of Python list - a tuple - which is ordered and unchangeable.

  • Option 4 gives you the most flexibility, as it allows you to customize the headers and other aspects of the response.

For our REST API, we will want to return JSON-formatted data. We will use a special Flask method to convert our list to JSON - flask.jsonify. (More on this later.)

Tip

Refer back to the Working with JSON material for a primer on the JSON format and relevant JSON-handling methods.

EXERCISE 2

Try doing this exercise in a Python (or ipython) shell.

  1. Serialize the list returned by the get_data() method above into a JSON-formatted string using the Python json library. Verify that the type returned is a string.

  2. Next, deserialize the string returned in part 1 by using the json library to decode it. Verify that the result equals the original list.

Returning JSON (and Other Kinds of Data)

You probably are thinking at this point we can fix our solution to the first Exercise by using the json library (which function?). Let’s try that and see what happens:

EXERCISE 3

Update your code from the first Exercise to use the json library to return a properly formatted JSON string.

Then, with your API server running in one window, open a Python3 interactive session in another window and:

  • Make a GET request to your /degrees URL and capture the response in a variable, say r

  • Verify that r.status_code is what you expect (what do you expect it to be?)

  • Verify that r.content is what you expect.

  • Use r.json() to decode the response and compare the type to that of r.content.

HTTP Content Type Headers

Requests and responses have headers which describe additional metadata about them. Headers are key: value pairs (much like dictionary entries). The key is called the header name and the value is the header value.

There are many pre-defined headers for common metadata such as specifying the size of the message (Content-Length), the domain the server is listening on (Host), and the type of content included in the message (Content-Type).

We can use curl or the python requests library to see all of the headers returned on a response from our flask server. Let’s try it.

EXERCISE 4

  1. Use curl to make a GET request to your /degrees endpoint and pass the -v (for “verbose”) option. This will show you additional information, including the headers. Note that with -v, curl shows headers on both the request and the response. Request headers are lines that start with a > while response headers are lines that start with a <.

  2. Use curl again to make the same request, but this time pass the --head option instead of the -v; this will show you only the headers being returned in the response.

  3. Inside a Python shell, use requests to make the same GET request to your /degrees endpoint, and capture the result in a variable, r. Inspect the r.header attribute. What is the type of r.headers?

curl localhost:5000/degrees -v

* About to connect() to localhost port 5000 (#0)
*   Trying ::1...
* Connection refused
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 5000 (#0)
> GET /degrees HTTP/1.1
> User-Agent: curl/7.29.0
> Host: localhost:5000
> Accept: */*
>
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Content-Type: text/html; charset=utf-8
< Content-Length: 210
< Server: Werkzeug/2.0.3 Python/3.6.8
< Date: Fri, 04 Mar 2022 01:12:34 GMT

In [1]: import requests

In [2]: r = requests.get('http://127.0.0.1:5000/degrees')

In [3]: r.headers
Out[3]: {'Content-Type': 'text/html; charset=utf-8', 'Content-Length': '210', 'Server': 'Werkzeug/2.0.3 Python/3.6.8', 'Date': 'Fri, 04 Mar 2022 01:21:41 GMT'}

We see that we are sending a Content-type of 'text/html. In some ways, that is true, but we can do better; we can tell the client we are sending json data.

Media Type (or Mime Type)

The allowed values for the Content-Type header are the defined media types (formerly, mime types). The main thing you want to know about media types are that they:

  • Consist of a type and subtype

  • The most common types are application, text, audio, image, and multipart

  • The most common values (type and subtype) are application/json, application/xml, text/html, audio/mpeg, image/png, and multipart/form-data

Content Types in Flask

The Flask library has the following built-in conventions you want to keep in mind:

  • When returning a string as part of a route function in Flask, a Content-Type of text/html is returned.

  • To convert a Python object to a JSON-formatted string and set the content type properly, use the flask.jsonify() function.

For example, the following code will convert the list to a JSON string and return a content type of aplication/json:

return flask.jsonify(['a', 'b', 'c'])

EXERCISE 5

Use the flask.jsonify() method to update your code from Exercise 1. Then:

  • Validate that your /degrees endpoint works as expected by using the requests library to make an API request and check that the .json() method works as expected on the response.

  • Use the .headers() method on the response to verify the Content-Type is what you expect.

Query Parameters

The HTTP specification allows for parameters to be added to the URL in form of key=value pairs. Query parameters come after a ? character and are separated by & characters; for example, the following request to a hypothetical API:

GET https://api.example.com/degrees?limit=3&offset=2

passes two query parameters: limit=3 and offset=2. Note that the URL path in the example above is still /degrees; that is, the ? character terminates the URL path, and any characters that follow create the query parameter set for the request.

In REST architectures, query parameters are often used to allow clients to provide additional, optional arguments to the request.

Common uses of query parameters in RESTful APIs include:

  • Pagination: specifying a specific page of results from a collection

  • Search terms: filtering the objects within a collection by additional search attributes

  • Other parameters that might apply to most if not all collections such as an ordering attribute (ascending vs descending)

Extracting Query Parameters in Flask

Flask makes the query parameters available on the request.args object, which is a “dictionary-like” object. To work with the query parameters supplied on a request, you must import the Flask request object, and use the args.get method to extract the passed query parameter into a variable.

Note

The flask.request object is different from the Python3 requests library we used to make http requests. the flask.request object represents the incoming request that our flask application server has received from the client.

1from flask import Flask, request
2
3@app.route('/degrees', methods=['GET'])
4def degrees():
5    start = request.args.get('start')
6    # additional code...

The start variable will be the value of the start parameter, if one is passed, or it will be None otherwise:

GET https://api.example.com/degrees?start=2

Note

request.args.get() will always return a string, regardless of the type of data being passed in.

Let’s use this idea to update our degrees_api to only return the years starting from the start query parameter year, if that parameter is provided.

@app.route('/degrees', methods=['GET'])
def degrees():
   d = get_data()
   start = int(request.args.get('start', 0))
   return flask.jsonify(d)

Error Handling

This brings up the topic of error handling. What happens if the user sends a value for the start query parameter that isn’t an integer? We can test it ourselves.

[isp02]$ curl 127.0.0.1:5000/degrees?start=abc

If we try this we get some nasty stuff that ends with a traceback, like this:

Traceback (most recent call last):
File "/home/jstubbs/.local/lib/python3.6/site-packages/flask/app.py", line 2091, in __call__
   return self.wsgi_app(environ, start_response)
File "/home/jstubbs/.local/lib/python3.6/site-packages/flask/app.py", line 2076, in wsgi_app
   response = self.handle_exception(e)
File "/home/jstubbs/.local/lib/python3.6/site-packages/flask/app.py", line 2073, in wsgi_app
   response = self.full_dispatch_request()
File "/home/jstubbs/.local/lib/python3.6/site-packages/flask/app.py", line 1518, in full_dispatch_request
   rv = self.handle_user_exception(e)
File "/home/jstubbs/.local/lib/python3.6/site-packages/flask/app.py", line 1516, in full_dispatch_request
   rv = self.dispatch_request()
File "/home/jstubbs/.local/lib/python3.6/site-packages/flask/app.py", line 1502, in dispatch_request
   return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/home/jstubbs/coe332-sp22/flask_2/degrees_api_2.py", line 31, in degrees3
   start = int(request.args.get('start', 0))
ValueError: invalid literal for int() with base 10: 'abc'

Checking User Input

If we get a request like this, in the best case, the user didn’t understand what kind of data to put in the start query parameter; in the worst case, they were intentionally trying to send our program bad data to break it. We should always be very careful with user-supplied data and make sure it contains the kind of data we expect.

So, what is it we expect from the start query parameter? At a minimum, it needs to be some kind of integer data, because we are casting it to the int type. Therefore, at a minimum, we should check if it is an integer.

We can use the Python isnumeric() method on a Python string to test whether a string contains non-negative integer data.

Let’s try some examples in the Python shell:

>>> '123'.isnumeric()
True
>>> 'abc'.isnumeric()
False
>>> '1.23'.isnumeric()
False
>>> '-1'.isnumeric()
False

Now, let’s fix our route function; we can check if it is numeric before casting to an int. If it is not numeric, we can return an error message to the user.

1 @app.route('/degrees', methods=['GET'])
2 def degrees():
3     d = get_data()
4     start = request.args.get('start', 0)
5     if not start.isnumeric():
6         return 'Invalid start value; start must be numeric.'
7     start = int(start)
8     return flask.jsonify(d[start:])

Exceptions

Using the isnumeric() function allowed us to check for invalid user input in the specific case above, but Python provides a far more general and powerful error handling capability, called Exceptions, that we will discuss next.

In Python, exceptions are the mechanism one typically uses to communicate and deal with run-time errors. Exceptions are different from syntax errors where, in general, there is no hope of the code working. Exceptions occur with statements that are syntactically correct but nonetheless generate some kind of error at runtime. Typically, the program can recover from these types of errors.

In Python, exceptions are instances of the class Exception or a child class. We say that a statement generates or raises an exception.

Some common situations that generate exceptions are:

  • Trying to open a file that does not exist raises a FileNotFoundError.

  • Trying o divide by zero raises a ZeroDivisionError.

  • Trying to access a list at an index beyond its length raises an IndexError.

  • Trying to use an object of the wrong type in a function raises a TypeError (for example, trying to call json.dumps() with an object that is not of type str.)

  • Trying to use an object with the wrong kind of value in a function raises a ValueError (for example, calling int('abc').)

  • Trying to access a non-existent attribute on an object raises an AttributeError (a special case is accessing a null/uninitialized object, resulting in the dreaded AttributeError: 'NoneType' object has no attribute 'foo' error.)

Handling Exceptions

If a statement we execute in our code, such as a call to the int() function to cast an object to an integer, could raise an exception, we can handle the exception by using the try...except... statement. It works like this:

try:
    # execute some statements that could raise an exception...
    f(x, y, z)
except ExceptionType1 as e:
    # do something if the exception was of type ExceptionType1...
except ExceptionType2 as e:
    # do something if the exception was of type ExceptionType2...

# . . . additional except blocks . . .

finally:
    # do something regardless of whether an exception was raised or not.

A few notes:

  • If a statement(s) within the try block does not raise an exception, the except blocks are skipped.

  • If a statement within the try block does raise an exception, Python looks at the except blocks for the first one matching the type of the exception raised and executes that block of code.

  • The finally block is optional but it executes regardless of whether an exception was raised by a statement or not.

  • The as e clause puts the exception object into a variable (e) that we can use.

  • The use of e was arbitrary; we could choose to use any other valid variable identifier.

  • We can also leave off the as e part altogether if we don’t need to reference the exception object in our code.

Here’s how we could deal with an invalid start parameter provided by the user using exceptions:

try:
    start = int(start)
except ValueError:
    # return some kind of error message...

# at this point in the code, we know the int(start) "worked" and so we are safe
# to use it as an integer..

Exception Hierarchy

Exceptions form a class hierarchy with the base Exception class being at the root. So, for example:

  • FileNotFoundError is a type of OSError as is PermissionError, which is raised in case the attempted file access is not permitted by the OS due to lack of permissions.

  • ZeroDivisionError and OverflowError are instances of ArithmeticError, the latter being raised whenever the result of a calculation exceeds the limits of what can be represented (try running 2.**5000 in a Python shell).

  • Every built-in Python exception is of type Exception.

Therefore, we could use any of the following to deal with a FileNotFoundError:

  • except FileNotFoundError

  • except OSError

  • except Exception

Here are some best practices to keep in mind for handling exceptions:

  • Put a minimum number of statements within a try block so that you can detect which statement caused the error.

  • Similarly, put the most specific exception type in the except block that is appropriate so that you can detect exactly what went wrong. Using except Exception... should be seen as a last resort because an Exception could be any kind of error.

Here is the full code for our route function with exception handling.

 1@app.route('/degrees', methods=['GET'])
 2def degrees():
 3   d = get_data()
 4   start = request.args.get('start')
 5   if start:
 6      try:
 7          start = int(start)
 8      except ValueError:
 9          return "Invalid start parameter; start must be an integer."
10   return flask.jsonify(d[start:])

EXERCISE 6

Add support for a limit parameter to the code you wrote for Exercise 5. The limit parameter should be optional. When passed with an integer value, the API should return no more than limit data points.

Additional Resources