Selenium has a pretty easy method of how to upload a file. You need to find a file input element and put a file path via send keys method. That’s it! But what’s about file size? Usually, it is not important, but sometimes size matters. Why?

So, let’s look what’s under the hood while Selenium uploads the files.

Please note. I’ll use Python for samples below, but you can apply to any programming language that is supported by Selenium.

File upload mechanism

Sample code looks like

from selenium.webdriver import Firefox

browser = Firefox()
# opening a page with upload window
browser.find_element_by_css_selector('fileinput').send_keys('path/to/my.file'))

What happens? Selenium finds the desired WEB element and assigns a text attribute with a given value (path/to/my.file). That’s all! Meanwhile, Remote WebDriver behaves differently. How?

Here is the previous code snippet, modified to support remote WebDriver:

from selenium.webdriver import FirefoxOptions, Remote

browser = Remote(
    command_executor="http://localhost:4444",
    desired_capabilities=FirefoxOptions().to_capabilities()
)
# opening a page with upload window
browser.find_element_by_css_selector('fileinput').send_keys('path/to/my.file'))

Selenium checks if an instance of a WebDriver is a remote one. If yes, it starts transferring of a given file (path/to/my.file) to a server, where a Firefox instance is running, or to a Selenium Grid node. And if the transferring is successful, Selenium assigns a correct value (some temporary path) to a text attribute of the WEB element. So, you don’t have to care about the presence of the file on a remote host. UX matters, isn’t it?

But what if you need to upload a file with a size of several gigabytes? It may happen that the file transport to remove server (node) will fail due to memory limitations, network issues, etc. For instance, here is a <Errno 32 broken pipe> in Python’s code

How to configure a large file upload with Selenium

The best option is to split this process into two stages:

1. Deliver a file to a server (node) where a browser is running
2. Let Selenium use prepared file

Stage 1. Deliver a file

There is a bunch of ways how to do that. For instance, you can copy a file over scp before running a test. Or, if you run your tests using dockerized browser via docker-compose, you can mount a volume to a browser’s container which will be responsible for files sharing.

Stage 2. Let Selenium use prepared file

There is an object called file detector which can disable (or enable) transferring of a file to a server while Remote WebDriver is used. Standard Python’s library provides LocalFileDetector and UselessFileDetector classes in selenium.webdriver.remote.file_detector module. The first one is a default one and enables a file transfer. The second one disables file transfer. So, UselessFileDetector has to be configured for a WebDriver instance. There are two options.

The first option is to configure it while creating an instance of Remote WebDriver

browser = Remote(
    command_executor="http://localhost:4444",
    desired_capabilities=FirefoxOptions().to_capabilities(),
    file_detector=UselessFileDetector()
)

It’s is a preferred way as it configures a global state of file uploads logic. However, if you want to turn off the file transferring only for some files, you may use a context manager like

with browser.file_detector_context(UselessFileDetector):
    browser.find_element_by_css_selector('fileinput').send_keys('path/to/my.file'))

Conclusion

Looks like UselessFileDetector is not a very good name for a class…

Anyway, sometimes useless can be very useful.