URL parameters with angle brackets in Django - Step-by-Step Execution

Concept Flow - URL parameters with angle brackets

Start: URL request

↓

Django URL dispatcher reads pattern

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Detect angle brackets <param>

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Extract parameter value from URL

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Pass parameter to view function

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View uses parameter to generate response

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Send response back to browser

Django reads the URL pattern with angle brackets, extracts the parameter from the URL, and passes it to the view function.

Execution Sample

Django

from django.urls import path
from django.http import HttpResponse

def view(request, name):
    return HttpResponse(f"Hello, {name}!")

urlpatterns = [
    path('hello/<str:name>/', view),
]

This code defines a URL pattern that captures a string parameter 'name' from the URL and passes it to the view.

Execution Table

Step	URL Requested	URL Pattern Matched	Parameter Extracted	View Called With	Response Output
1	/hello/Alice/	hello/<str:name>/	name = 'Alice'	view(request, name='Alice')	Hello, Alice!
2	/hello/Bob/	hello/<str:name>/	name = 'Bob'	view(request, name='Bob')	Hello, Bob!
3	/hello/123/	hello/<str:name>/	name = '123'	view(request, name='123')	Hello, 123!
4	/hello/	No match	No parameter	No view called	404 Not Found

💡 Execution stops when URL does not match the pattern or after response is sent.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
name	None	'Alice'	'Bob'	'123'	None

Key Moments - 3 Insights

Why does the URL '/hello/' not call the view function?

Because the URL pattern expects a parameter inside angle brackets, like '<str:name>', so '/hello/' without a parameter does not match the pattern (see execution_table row 4).

What happens if the parameter is a number but the pattern expects a string?

The pattern '<str:name>' accepts any string including numbers as strings, so '123' is accepted as a string parameter (see execution_table row 3).

How does Django know which part of the URL is the parameter?

Django uses the angle brackets in the URL pattern to identify the parameter position and type, then extracts that part from the URL (see concept_flow step 'Detect angle brackets <param>').

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'name' when the URL '/hello/Bob/' is requested?

A 'Alice'

B 'Bob'

C '123'

D None

💡 Hint

Check execution_table row 2 under 'Parameter Extracted' and 'View Called With'.

At which step does the URL not match the pattern and cause a 404 error?

A Step 4

B Step 3

C Step 1

D Step 2

💡 Hint

Look for 'No match' and '404 Not Found' in execution_table.

If the URL pattern was changed to 'hello/<int:name>/', what would happen when requesting '/hello/Alice/'?

A The view is called with name='Alice'

B The view is called with name=0

C The URL does not match and returns 404

D The server crashes

💡 Hint

Check how type converters like affect matching in the execution_table and concept_flow.

Concept Snapshot

Django URL patterns use angle brackets <param> to capture parts of the URL as parameters.
Syntax: path('route/<type:name>/', view).
The parameter is extracted from the URL and passed to the view.
If the URL does not match the pattern, Django returns 404.
Type converters like str, int specify expected parameter type.

Full Transcript

In Django, URL patterns can include parameters inside angle brackets, like <str:name>. When a URL request comes in, Django matches it against the pattern. If it matches, Django extracts the parameter value from the URL and passes it to the view function as an argument. For example, the URL '/hello/Alice/' matches the pattern 'hello/<str:name>/', so the parameter 'name' is set to 'Alice'. The view then uses this parameter to create a response, such as 'Hello, Alice!'. If the URL does not include the parameter or does not match the pattern, Django returns a 404 error. Type converters like str or int inside the angle brackets tell Django what type to expect and help with matching. This process allows dynamic URLs that can change based on user input or other data.