Web apps with Flask: cheat sheet

Web apps with Flask · In any playground, open Reference to copy these.

Pages from Python

home

Go back to the centre, facing right.

home()

Example

forward(80)
left(90)
forward(40)
home()

What it draws — Back to the centre, facing right

variable

Give a value a name, so you can reuse and change it.

size = 50

Example

length = 120
forward(length)
left(90)
forward(length / 2)

list

Keep many values in order under one name. The first item is place 0.

colours = ["red", "blue", "green"]

for each item

Work through a list, one item at a time.

for c in colours:
    pass

Example

pensize(6)
for c in ["red", "blue", "green"]:
    pencolor(c)
    forward(40)

join text

Join pieces of text into one with +. Spaces only appear where they are inside the quotes.

"Hello, " + name

dictionary value

The value stored under that key. The key goes in quotes. Assign to it to change it: star["speed"] = 8.

star["speed"]

in

True if the item is in the list (or the key is in the dictionary, or the letters are in the text).

if item in bag:
    pass

a page from a template

A template is HTML with gaps: {{ }} shows a value, {% for %} repeats a part for each item.

PAGE = """<ul>
{% for m in members %}
  <li>{{ m["name"] }}</li>
{% endfor %}
</ul>"""

return render_template_string(PAGE, members=members)

if / else

Do one thing or the other, depending on a condition.

if x > 0:
    pass
else:
    pass

Example

for x in [-90, -30, 30, 90]:
    penup()
    goto(x, 0)
    if x > 0:
        dot(30, "red")
    else:
        dot(30, "blue")
hideturtle()

grow value

Change a value every pass: read the old one, store a new one.

size = 5
for i in range(20):
    forward(size)
    right(90)
    size = size + 5

Example

size = 5
for i in range(30):
    forward(size)
    right(90)
    size = size + 5

def (function)

Name a group of steps, then run it by calling its name.

def shape():
    pass

shape()

Example

def zigzag():
    left(45)
    forward(30)
    right(90)
    forward(30)
    left(45)

zigzag()
zigzag()
zigzag()

a Flask route

Joins an address to a function. What the function returns is the page.

@app.route("/about")
def about():
    return "<h1>About</h1>"

an address with a part that changes

<name> matches any word in that place of the address, and is passed to the function.

@app.route("/group/<name>")
def group(name):
    return "You asked for  + name

Forms and saving

print

Show text or a value in the output box.

print("Hello")

write a file

"w" makes the file, or empties it first if it is already there, then writes. with closes the file at the end.

with open("save.txt", "w") as f:
    f.write(text)

read a CSV

Each row of the file becomes a dictionary, keyed by the column names in the first line. Every value is text.

import csv

with open("music.csv") as f:
    rows = list(csv.DictReader(f))

go to another page

Sends the browser on to another address, such as back home after a form.

return redirect("/")

strip and lower

strip() removes spaces at the ends; lower() and upper() change the capitals. Each gives back new text.

command = command.strip().lower()

read a file

Opens the file to read and gives you all of it as one piece of text.

with open("save.txt") as f:
    text = f.read()

read a form

A form with method='post' sends each box that has a name='...' attribute. request.form looks them up by that name.

@app.route("/join", methods=["POST"])
def join():
    name = request.form["name"].strip()

add a row to a CSV

"a" adds to the end of the file instead of emptying it. Each list becomes one row.

with open("members.csv", "a", newline="") as f:
    csv.writer(f).writerow([name, group])

APIs

send JSON

Return a dictionary and Flask sends it as JSON, for other programs to read.

@app.route("/api/members")
def api_members():
    return {"members": members}

a FastAPI route

The type after each parameter (int, str, float) is checked: a wrong value gets an error reply (422) before your code runs.

@api.get("/year")
def year(y: int):
    return {"next": y + 1}