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Minimal working example
Install Pyppeteer, launch Chromium, create a page, navigate, evaluate JavaScript, print the result, and close the browser:
import asyncio
from pyppeteer import launch
async def main():
browser = await launch()
page = await browser.newPage()
await page.goto('https://example.com')
title = await page.evaluate('''() => document.title''')
greeting = await page.evaluate('''(name) => `Hello, ${name}`''', 'Ada')
print(title, greeting)
await browser.close()
asyncio.get_event_loop().run_until_complete(main())The important details are the await before page.evaluate(), the JavaScript string, and the arguments supplied after the function string. Evaluation occurs inside the loaded page, so browser globals such as document, window, and the DOM are available.
What page.evaluate() does
Pyppeteer’s API executes a JavaScript function or expression in the browser and returns its result to Python. Simple values and JSON-compatible objects are serialized across the browser boundary. A returned object such as {width, height} becomes a Python dictionary; a returned array becomes a Python list.
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Read page dimensions
dimensions = await page.evaluate('''() => {
return {
width: document.documentElement.clientWidth,
height: document.documentElement.clientHeight,
deviceScaleFactor: window.devicePixelRatio,
}
}''')
print(dimensions)The values describe the current page and viewport, not the dimensions of an image or PDF produced later. Set the viewport before evaluating if your result depends on responsive layout.
Function strings and expression strings
Pyppeteer accepts JavaScript as a string and attempts to determine whether the string is a callable function or a direct expression. Arrow functions and regular functions are the clearest form when you need parameters, statements, or a return value:
result = await page.evaluate('''(a, b) => {
const total = a + b
return total * 2
}''', 2, 3)
print(result) # 10A direct expression is useful for a single property or calculation:
content = await page.evaluate('document.body.textContent', force_expr=True)
Use the keyword-only option force_expr=True when Pyppeteer misidentifies an expression as a function. This is especially useful for strings beginning with an identifier, property access, or another expression that does not look like an arrow or regular function.
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- Function string: use
() => ...or(value) => ...for arguments, multiple statements, conditionals, and explicit returns. - Expression string: use a short expression such as
document.title; addforce_expr=Trueif detection is ambiguous. - Async browser code: make the callback async when it must await a promise, then await the Python call as usual.
data = await page.evaluate('''async () => {
const response = await fetch('/api/status')
return await response.json()
}''')The page’s origin, cookies, permissions, and content-security behavior still apply to code running in that page. A failed fetch or rejected promise is a browser-side failure that Pyppeteer reports back to Python.
Passing arguments safely
Every positional value after the JavaScript string is serialized and supplied to the callback in order:
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result = await page.evaluate(
'''(a, b) => a + b''',
2,
3,
)
Pass structured data as normal Python dictionaries and lists when their contents are serializable:
config = {'selector': 'h1', 'uppercase': True}
text = await page.evaluate('''(options) => {
const node = document.querySelector(options.selector)
if (!node) return null
const value = node.textContent.trim()
return options.uppercase ? value.toUpperCase() : value
}''', config)
Do not pass Python functions, open files, sockets, or arbitrary class instances as callback arguments. Convert them to strings, numbers, booleans, lists, or dictionaries first.
Evaluating against a selected element
Find an element handle, then pass that handle as the first callback argument:
element = await page.querySelector('h1')
if element is None:
raise RuntimeError('No h1 element found')
text = await page.evaluate(
'''(element) => element.textContent''',
element,
)
print(text)The handle refers to a DOM node in the page. It is not a copy of the element, and it becomes invalid if navigation replaces the document.
Use querySelectorEval() for one operation
querySelectorEval(selector, pageFunction, *args) combines selection and evaluation. The matching element is passed as the first callback argument:
text = await page.querySelectorEval(
'h1',
'''(element, suffix) => element.textContent.trim() + suffix''',
'!',
)If no element matches, this method raises an element error. Use an explicit querySelector() check when “not found” is an expected outcome that your code should handle.
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Related Pyppeteer APIs
| API | Use it when | Result or timing |
|---|---|---|
evaluate() |
You need a one-off calculation, DOM read, mutation, or returned value. | Returns a serialized value immediately after the JavaScript finishes. |
evaluateHandle() |
The result is an in-page object you will inspect or manipulate through the DevTools protocol. | Returns a persistent JSHandle, not a copied Python value. |
evaluateOnNewDocument() |
Code must be installed before navigation or in newly attached child frames. | Registers code that runs for future document loads. |
waitForFunction() |
You need to wait until a browser-side condition becomes truthy. | Polls a predicate rather than performing a single evaluation. |
Persistent handles
handle = await page.evaluateHandle('''() => window.localStorage''')
# Use handle with other Pyppeteer operations, then dispose it when finished.
await handle.dispose()Prefer evaluate() when you only need data in Python. Handles keep a reference in the browser and should be disposed when no longer needed.
Inject code before a page loads
await page.evaluateOnNewDocument('''() => {
window.myAutomationFlag = true
}''')
await page.goto('https://example.com')
This is different from calling evaluate() after navigation: the registered script runs during subsequent document creation, including child frames.
Waiting for dynamic content
Evaluation sees the DOM at the instant it runs. For client-rendered pages, wait for a selector or a predicate first:
await page.waitForSelector('.product-card')
count = await page.evaluate('''() => document.querySelectorAll('.product-card').length''')
await page.waitForFunction('''() => {
return document.querySelectorAll('.product-card').length >= 10
}''')
Use a selector wait when a specific node signals readiness. Use waitForFunction() for a value that must become true, such as a minimum item count or a global application flag. Add an explicit timeout appropriate to the site so a broken page does not wait forever.
Common errors and fixes
“Evaluation failed” or syntax errors
- Check the JavaScript string for unmatched quotes, braces, or backticks.
- Use a function form with an explicit
returnfor multi-line code. - Run a plain property expression with
force_expr=True.
The result is None
JavaScript returns undefined when a function has no return statement. Add return value; also check that a selector actually matched an element.
Element handle errors
A handle can be None because the selector matched nothing, or stale because navigation replaced the document. Wait for the selector and reacquire the handle after every navigation.
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Timeouts
Evaluation itself is usually quick, but JavaScript that waits on a network request or an unresolved promise can exceed the page timeout. Add error handling around the call, set a bounded timeout, and verify that the page’s request is reachable from the browser context.
Cross-origin frames
An element belongs to its own frame. Obtain the appropriate frame and evaluate there rather than assuming the main page document contains the node. Browser security rules still prevent unrestricted access across origins.
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DOM nodes, functions, symbols, and cyclic objects cannot be returned as ordinary JSON data. Return the specific fields you need, such as textContent, attributes, or an array of plain objects.
Reliability and performance practices
- Navigate first and wait for the application state your script requires.
- Keep callbacks small; perform heavy parsing in Python after returning compact data.
- Return plain serializable values instead of large DOM subtrees.
- Reuse a browser and page for related operations, but close both in a
finallyblock during failures. - Release
JSHandleobjects withdispose(). - Make selectors specific and resilient to cosmetic class-name changes.
- Log the URL, selector, and a short error message when an evaluation fails; avoid logging credentials or page secrets.
async def read_title(url):
browser = await launch()
try:
page = await browser.newPage()
await page.goto(url, {'waitUntil': 'networkidle2'})
return await page.evaluate('''() => document.title''')
finally:
await browser.close()The exact navigation wait setting should match the application. Network-idle waits can be unsuitable for pages with continuous analytics or streaming requests; a specific selector or predicate is often more deterministic.
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FAQ
Can I call a Python function inside the page?
No. The callback executes JavaScript in Chromium. Pass data from Python as serialized arguments, then return serialized results.
Does evaluate() automatically wait for a selector?
No. Call waitForSelector() or waitForFunction() before evaluating dynamic content.
When should I use querySelectorEval()?
Use it for a concise selector-plus-callback operation when a missing element should raise an error.
Frequently Asked Questions
Can I call a Python function inside the page?
No. The callback executes JavaScript in Chromium. Pass data from Python as serialized arguments, then return serialized results.
Does evaluate() automatically wait for a selector?
No. Call waitForSelector() or waitForFunction() before evaluating dynamic content.
When should I use querySelectorEval()?
Use it for a concise selector-plus-callback operation when a missing element should raise an error.
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