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README.md mar 23 lecture Mar 21, 2020
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README.md

March 23 Lecture

We'll wrapup web scraping in the first two videos, then start talking about advanced plotting is the third video.

1. Response Objects

Watch: 11-minute video

Practice: jsonify

To understand how jsonify works, here you'll write your own my_jsonify that behaves the same way. This function does three things at once:

  1. converts data (Python dict, list, etc) to a string containing JSON that can be used as the response body
  2. creates a Response object using the string from step 1
  3. sets the "Content-Type" header to "application/json"

Paste the following example to a .py file, and complete my_jsonify.

from flask import Flask, request, Response
import json

app = Flask(__name__)

def my_jsonify(data):
    resp_body = json.dumps(????)
    r = Response(resp_body, headers={"????": "????"})
    return r

@app.route("/math.json")
def home():
    x = float(request.args.get("x", 0))
    y = float(request.args.get("y", 0))
    results = {"add": x+y, "sub": x-y, "mult": x*y}
    return my_jsonify(results)

if __name__ == "__main__":
    app.run("0.0.0.0")

Try going to http://YOUR-IP:5000/math.json?x=5&y=3 in Chrome to see different math operators applied to 5 and 3. Open the dev tools and check that you're getting "application/json" for the "Content-Type" in addition to the correct answers. It ought to look like this:

2. Rate Limiting

Watch: 21-minute video

Practice: compliant crawling

Copy/paste this example from a video to a fruits.py file on your VM and run it:

from flask import Flask, request, Response
import json, time

app = Flask(__name__)

fruits = ["apple", "banana", "kiwi", "cantaloupe", "berries", "orange"]

# key: client IP addr
# val: last time request served (in seconds since 1970)
last_req = {}

def rate_limit(fn):
    def wrapper():
        # policy: on request every 2 seconds, per IP address
        client_ip = request.remote_addr
        next_allowed = last_req.get(client_ip, 0) + 2
        now = time.time()
        
        should_allow = now >= next_allowed
        if not should_allow:
            return Response("backoff!!", status=429, 
                            headers={"Retry-After": next_allowed-now})
        last_req[client_ip] = now
        return fn()
    wrapper.__name__ = fn.__name__
    return wrapper

@app.route("/fruit")
@rate_limit
def fruit():
    idx = int(request.args.get("idx", 0))
    if idx >= len(fruits):
        return ""
    return fruits[idx]

if __name__ == "__main__":
    app.run("0.0.0.0")

Now paste and finish the fruit scraper from the video example to a notebook. After entering your IP, try running it and looking carefully at how r.headers looks when printed before figuring out how long to sleep.

import requests, time

my_ip = "????" # TODO: put your VM's IP addr here

def nice_get(url):
    print("GET", url)
    r = requests.get(url)
    if r.status_code == 429:
        # told to backoff...
        # we'll wait however long requested, then try once more

        print("Response Headers Dict:", r.headers)

        # TODO: pull "Retry-After" from headers,
        # convert to float, and pass to sleep call:

        # time.sleep(????)
        r = requests.get(url)
    r.raise_for_status()
    return r.text

def fruit_url(idx):
    return ("http://"+my_ip+":5000"+
            "/fruit?idx=" + str(idx))

def grab_fruits():
    fruits = []
    i = 0
    while True:
        url = fruit_url(i)
        fruit = nice_get(url)
        if fruit != "":
            fruits.append(fruit)
        else:
            return fruits
        i += 1
        
grab_fruits()

3. Matplotlib Coordinate Systems

Watch: 13-minute video

Practice: scatter, from scratch

Paste+run the following:

import pandas as pd

df = pd.DataFrame([
    {"x":0.1, "y":0.4},
    {"x":0.2, "y":0.2},
    {"x":0.3, "y":0.1},
    {"x":0.4, "y":0.3}
])
df

Now paste+run this:

fig, (ax1, ax2) = plt.subplots(ncols=2, figsize=(7, 4))
ax2.set_xlim(0.1, 0.6)

points1 = ax1.transData.transform(df[["x", "y"]].values) / fig.dpi
print(points1)

def scatter(ax, points):
    for x, y in points:
        p = plt.Circle((x, y), 0.1, facecolor="blue",
                       transform=fig.dpi_scale_trans)
        ax.add_artist(p)

scatter(ax1, points1)

AxesSubplot.transData.transform (not covered in lecture) can convert an array of points from the coordinate system of the AxesSupblot to absolute coordinates, in DPI. Dividing by fig.dpi then gives us inches, which are used by the above scatter method.

Can you modify the above code so it draws the same scatter points as black dots on the right? It ought to look like this:

You'll need to (1) make another .transform call based on the ax2 coordinate system, (2) make an additional call to scatter, and (3) add a parameter to scatter to control the facecolor.

Note: after I made this lecture, I realized the scatter plots in the picture weren't quite placed correctly. I'll talk about why in the next lecture.

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