Common Mistakes🔖

These are the mistakes nearly everyone makes in Week 1. Knowing about them in advance won't prevent all of them, but it might save you 20 minutes of confusion.

Using print() instead of return🔖

# WRONG — shows in terminal, browser gets nothing
@app.route("/")
def index():
    print("<h1>Hello</h1>")
 
# RIGHT — sends HTML to the browser
@app.route("/")
def index():
    return "<h1>Hello</h1>"

Forgetting the @ in the decorator🔖

# WRONG — this is just a function call, not a decorator
app.route("/")
def index():
    return "<h1>Hello</h1>"
 
# RIGHT
@app.route("/")
def index():
    return "<h1>Hello</h1>"

Without the @, Python runs app.route("/") as a standalone expression (which does nothing useful) and index() becomes a regular function that Flask doesn't know about.

Naming two functions the same thing🔖

@app.route("/")
def page():
    return "<h1>Home</h1>"
 
@app.route("/about")
def page():          # <-- same name!
    return "<h1>About</h1>"

Python silently overwrites the first page with the second. The / route will break. Give each function a unique name that matches its purpose.

Returning plain text without HTML tags🔖

@app.route("/")
def index():
    return "Hello World"  # works, but looks plain and unstyled

This technically works — the browser shows "Hello World." But without <h1>, <p>, etc., it's just a wall of unstyled text. At minimum, wrap your content in basic tags.

TemplateNotFound error🔖

@app.route("/about")
def about():
    return render_template("abuot.html")  # typo!

The filename you pass to render_template() must exactly match a file inside templates/. Common causes:

• Typo in the filename
• File is in the wrong folder (sitting next to app.py instead of inside templates/)
• Wrong capitalization (About.html vs about.html)

Forgetting to save before reloading🔖

With --debug mode, Flask auto-reloads when you save. But it can't reload what you haven't saved. If your changes aren't appearing, check that you've actually saved the file (Cmd+S / Ctrl+S).

Broken image or no image🔖

Two different problems look similar but mean different things:

• Broken image icon (cracked rectangle shows in the page): Flask found the URL but the file isn't there. Check that the image is inside the static/ folder and that the filename matches exactly — including capitalization.
• Nothing visible at all: The <img> tag is missing from the HTML, or there's a typo in the tag. Use CMD-F and search for <img across your files to confirm it exists.

Common path mistake:

<!-- WRONG — Flask doesn't serve files from project root -->
<img src="photo.jpg" alt="...">
 
<!-- RIGHT — files in static/ are served at /static/ -->
<img src="/static/photo.jpg" alt="...">

The server "isn't working" (it's still running)🔖

If you see no prompt in the terminal, the server is still running. That's correct. It's waiting for requests. Don't close the terminal — open a new one or use the browser.

To stop the server intentionally: Ctrl+C.