Trailing slash behavior in Flask - Step-by-Step Execution

Concept Flow - Trailing slash behavior

Client requests URL

↓

Check route with trailing slash?

↓

Redirect to URL with slash

↓

Client follows redirect

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Return response

When a Flask route ends with a slash, Flask redirects requests without the slash to the slash version. If no slash, Flask serves directly or 404.

Execution Sample

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/hello/')
def hello():
    return 'Hello!'

# Request to '/hello' triggers redirect to '/hello/'

Defines a route with trailing slash; accessing URL without slash redirects to URL with slash.

Execution Table

Step	Requested URL	Route Defined	Match?	Action	Response
1	/hello	/hello/	No exact match	Redirect to '/hello/'	302 Redirect
2	/hello/	/hello/	Match	Call hello()	200 OK with 'Hello!'
3	/hello/	/hello	No route defined	404 Not Found	404 Error

💡 Execution stops after redirect or response is returned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
Requested URL	None	/hello	/hello/	/hello/
Matched Route	None	None (redirect)	/hello/	None
Response	None	302 Redirect	200 OK 'Hello!'	404 Not Found

Key Moments - 2 Insights

Why does Flask redirect from '/hello' to '/hello/' when the route ends with a slash?

Because the route is defined with a trailing slash, Flask expects the URL to have it. When missing, Flask sends a redirect to add the slash, as shown in execution_table step 1.

What happens if you define a route without a trailing slash and access it with a slash?

Flask does not redirect in this case and returns 404 Not Found, because the exact route with slash is not defined. This is the opposite behavior of trailing slash routes.

Visual Quiz - 3 Questions

Look at the execution_table, what response does Flask send when requesting '/hello' if the route is defined as '/hello/'?

A 302 Redirect to '/hello/'

B 200 OK with 'Hello!'

C 404 Not Found

D 500 Internal Server Error

💡 Hint

Check execution_table row 1 for requested URL '/hello' and route '/hello/'

At which step does Flask serve the actual content 'Hello!'?

A Step 3

B Step 1

C Step 2

D Never

💡 Hint

Look at execution_table row 2 where requested URL matches route exactly

If the route was defined as '/hello' (no slash), what would happen when requesting '/hello/'?

A Flask redirects to '/hello'

B Flask returns 404 Not Found

C Flask serves the route directly

D Flask returns 500 Internal Server Error

💡 Hint

Refer to key_moments about no trailing slash route behavior

Concept Snapshot

Flask routes ending with a slash expect URLs with slash.
If URL misses slash, Flask redirects (302) to add it.
Routes without trailing slash do not redirect if slash is added.
This helps avoid duplicate URLs and keeps URLs consistent.
Remember: trailing slash means 'folder-like' URL; no slash means 'file-like' URL.

Full Transcript

In Flask, when you define a route with a trailing slash like '/hello/', Flask expects the URL to end with a slash. If a user requests '/hello' without the slash, Flask sends a 302 redirect to '/hello/'. The client then follows this redirect and Flask serves the content. If the route is defined without a trailing slash, Flask will not redirect if the URL has a slash and will return a 404 error instead. This behavior helps keep URLs consistent and avoids duplicate content. The execution table shows these steps clearly: first a redirect when the slash is missing, then serving the content when the URL matches exactly, or a 404 if no matching route exists.