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URL path parameters are named values embedded in a route, such as /users/34. A router matches the fixed parts of the path, captures the value at the parameter position, and passes it to your handler. Use path parameters when the value identifies the resource being requested; use query parameters after ? for filtering, sorting, pagination, and other options.
For example, /users/:userId/books/:bookId in Express captures 34 and 8989 as req.params.userId and req.params.bookId. FastAPI writes the same idea as /users/{user_id}, while Django uses converters such as <int:user_id>.
What is a URL path parameter?
The URI path is the portion after the authority (for example, example.com) and before the first question mark, number sign, or the end of the URI. MDN defines it as the component “terminated by the first question mark (?), number sign (#), or the end of the URI” (MDN URI path reference).
A path parameter is a named variable inside that path. In /orders/8472, 8472 can be the order ID. In /teams/acme/members/17, both acme and 17 are parameters. Express describes these as “named URL segments that are used to capture the values specified at their position in the URL.” Captured values normally arrive as strings unless the framework converts them.
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Path parameter versus query parameter
| Concern | Path parameter | Query parameter |
|---|---|---|
| Location | /users/42 |
/users?role=admin |
| Typical meaning | Selects a specific resource or hierarchy | Changes a collection response or operation |
| Routing | Usually participates in route matching | Does not participate in an Express route path |
| Examples | ID, slug, year, file path | Filter, sort, page, limit |
Do not put a filter such as status=paid into a path parameter merely because it is convenient. A useful rule is: if removing the value changes which resource is addressed, it belongs in the path; if it changes how a collection is presented, it usually belongs in the query string.
Express route parameters
Declare and read named segments
Express uses a colon before each parameter name:
const express = require('express');
const app = express();
app.get('/users/:userId/books/:bookId', (req, res) => {
const { userId, bookId } = req.params;
res.json({ userId, bookId });
});
app.listen(3000);A request to /users/34/books/8989 produces { userId: "34", bookId: "8989" }. Convert and validate before using the values:
app.get('/users/:userId', (req, res) => {
const userId = Number.parseInt(req.params.userId, 10);
if (!Number.isInteger(userId) || userId < 1) {
return res.status(400).json({ error: 'userId must be a positive integer' });
}
// Check authorization and fetch the user here.
res.json({ userId });
});Wildcards, optional segments, and precedence
Current Express routing uses path-to-regexp v8. Named wildcards can capture trailing path segments when a resource may contain slashes. Optional segments are useful only when both URL shapes have a clear meaning; otherwise use separate routes for explicit behavior. Regular-expression characters are not supported inside string paths in the current routing guide, so do not assume an old Express pattern will continue to work.
Express selects the first matching route. Put fixed exceptions before broad parameters:
app.get('/book/create', createBook);
app.get('/book/:bookId', getBook);If the order is reversed, the word create can be captured as bookId. Query strings such as /book/12?format=short do not change which Express route matches.
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FastAPI path parameters
Brace syntax and automatic conversion
FastAPI declares variables with the same brace syntax used by Python format strings (FastAPI path-parameter documentation):
from fastapi import FastAPI
app = FastAPI()
@app.get('/items/{item_id}')
def read_item(item_id: int):
return {'item_id': item_id}Calling /items/3 passes the integer 3. Calling /items/not-a-number produces FastAPI’s validation response instead of entering the handler. Type annotations therefore provide conversion, validation, and generated interactive API documentation.
Static routes must come first
FastAPI evaluates path operations in declaration order. Declare /users/me before /users/{user_id}; otherwise me can be interpreted as a user ID:
@app.get('/users/me')
def current_user():
return {'name': 'signed-in user'}
@app.get('/users/{user_id}')
def user(user_id: int):
return {'user_id': user_id}Capturing slashes
The default variable captures one path segment. For a file-like value that can contain slashes, use Starlette’s path converter:
@app.get('/files/{file_path:path}')
def read_file(file_path: str):
return {'path': file_path}FastAPI notes that OpenAPI does not natively provide a way to declare a path parameter containing an entire path, because such definitions are difficult to test and describe. Document this behavior explicitly for clients.
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Django converters and URL patterns
Built-in converters
Django’s path() function makes the expected type part of the route:
from django.urls import path
from . import views
urlpatterns = [
path('articles/<int:year>/', views.year_archive),
path('posts/<slug:slug>/', views.post),
path('accounts/<uuid:id>/', views.account),
path('files/<path:file_path>/', views.file),
]str(the default) matches any non-empty string except/.intmatches a nonnegative integer and passes an integer to the view.slugaccepts ASCII letters and numbers plus hyphens and underscores.uuidmatches a formatted lowercase UUID.pathincludes slashes and can match a complete URL path.
When built-in converters are insufficient, register a custom converter. Use re_path() for a regular expression that cannot be expressed cleanly with a converter. A nonmatching converter normally results in Django’s 404 handling rather than a view call.
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Choose stable resource paths
- Use nouns and consistent hierarchy:
/customers/{customerId}/invoices/{invoiceId}. - Keep identifiers in the path when they select one resource.
- Use query parameters for collection controls such as
?page=2&sort=-created. - Pick one spelling and casing convention; changing a public path is an API compatibility event.
Treat every captured value as untrusted
Validation is not authorization. First enforce type, format, range, and allowed values; then verify that the authenticated caller may access the selected resource. Return a clear 4xx response for malformed input and avoid leaking whether sensitive identifiers exist.
One segment or many?
Decide whether a parameter may contain /. Encode reserved characters correctly, and test encoded separators, Unicode, empty values, and trailing slashes against the exact router and proxy configuration. A wildcard can be useful for storage keys, but it also creates ambiguity and makes caching and documentation harder.
Trailing slashes and decoding
Express, FastAPI/Starlette, Django, reverse proxies, and CDNs can differ on whether /items/3 and /items/3/ are equivalent and when percent-decoding occurs. Set a deliberate policy, test both forms, and avoid using a decoded slash as an identifier delimiter.
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Route precedence and common bugs
Dynamic routes swallow static routes
Symptom: /users/me reaches the numeric user handler. Fix: register the static route first (FastAPI and Express), or constrain the dynamic converter to integers (Django or a typed FastAPI parameter).
Everything arrives as text
Symptom: arithmetic or strict comparisons fail. Express gives strings, so parse and validate explicitly. FastAPI and Django converters can perform conversion before the handler.
Query data is mistaken for path data
Symptom: a route expecting /products/:id is called with /products?id=7. These are different URLs. Either change the client to /products/7 or define a query-based collection endpoint.
Encoded values do not match expectations
Symptom: spaces, Unicode, or slashes are truncated or decoded twice. Log the raw request and the framework’s captured value, then apply one consistent encoding/decoding policy. Never concatenate untrusted path text into filesystem paths or shell commands.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Testing and documenting parameters
- List every route shape, including static exceptions and wildcard forms.
- Test valid values, wrong types, minimum and maximum bounds, empty values, Unicode, percent-encoded reserved characters, and both trailing-slash forms.
- Assert the status code and error body for malformed and unauthorized requests.
- Test precedence explicitly: static paths must not be captured by dynamic parameters.
- Document each parameter’s name, type, format, allowed values, example, and whether it may contain slashes.
FastAPI derives OpenAPI documentation from declarations. Express and Django projects generally need an explicit OpenAPI schema or equivalent documentation. Keep examples synchronized with the actual router; a stale example is an integration bug.
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Browser-side matching with URLPattern
The browser URLPattern API matches URL components using literal text, wildcards such as /posts/*, named groups such as /books/:id, optional groups, and regular-expression groups. Its syntax is based on path-to-regexp. MDN labels URLPattern “Baseline 2025,” meaning support across the latest devices and browser versions since September 2025; verify compatibility before using it for older browsers. It is a client-side matching tool, not a replacement for server authorization or route dispatch.
Or skip the browser setup
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com/users/34/books/8989 -o shot.webpThe same request in Python:
import requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://example.com/users/34/books/8989"},
timeout=90,
)
open("shot.webp", "wb").write(r.content)And Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://example.com/users/34/books/8989' });
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Cost, performance, and operational notes
- Keep path parameters short and cache-friendly; very large wildcard values increase log, cache-key, and observability costs.
- Use stable IDs or slugs rather than mutable display names when links must survive renames.
- Reject invalid values at the edge or framework validation layer to avoid unnecessary database work.
- Do not log secrets embedded in paths. URLs often appear in access logs, browser history, analytics, and referrer headers.
- When changing a route, provide redirects or a versioned endpoint and update generated documentation and client tests together.
Quick framework reference
| Framework | Declaration | Captured value | Type behavior |
|---|---|---|---|
| Express | /users/:userId |
req.params.userId |
String by default; parse and validate |
| FastAPI | /users/{user_id} |
Handler argument | Python annotation converts and validates |
| Django | users/<int:user_id>/ |
View argument | Converter controls matching and conversion |
Frequently Asked Questions
Can a path parameter be optional?
Some routers support optional syntax, but separate explicit routes are usually clearer and easier to document. Check the exact version of your framework before relying on optional-segment behavior.
Should IDs be numeric or strings?
Use the identifier format that is stable for your domain. Validate its exact format and authorize access; do not assume a numeric-looking value is safe or meaningful without those checks.
Are path parameters visible to users?
Yes. They appear in the address bar and commonly in server logs, browser history, analytics, and referrer data. Never place passwords, access tokens, or other secrets in them.
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