Understanding get_image_files in fastai
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In fastai library we use Datablocks like the below example for loading datasets, and to train models. The below code is DataBlock, which is used to load a dataset of various types of bears to split into train and validation datasets along and to resize images to size 128*128. For a detailed explanation, check on From Data to DataLoaders section in .
In this Datablock get_items, we are using the get_image_files to load the images. I was curious how to see how get_image_files worked under the hood to return all the image files in a dataset. As always suggests, I started looking into source code by handy question mark functionality in Jupyter Notebooks. The source code for get_image_files can be found in . The source code for get_image_files function is:
You can see it's expecting the path to the folder where files are present in the image folder. Also the function signature, consists of recurse=True and folder=None by default.
You can see get_image_files function is calling get_files(path, extensions=image_extensions, recurse=recurse, folders=folders) on passing with extensions set as image_extensions.
What is image_extensions doing?
The image extensions is just a variable returning a set of images from the , which is part of Python standard library to map filenames to MIME types. Let's see image_extensions output to see whole set of image type extensions.
mimetypes.types_map.items() returns a dictionary items. It consists of key, value pair and we are selecting value pairs starting with the word image/ to return a set of image_extensions as shown in the above output.
Now let's look at get_files function, which returns a list of files, based on extensions passed, folders. Let's look into the source code of get_files(path, extensions=None, recurse=True, folders=None, followlinks=True function as well:
Let's understand what the function get_files(path, extensions=None, recurse=True, folders=None, followlinks=True) is doing line by line.Let's look at the first few lines of code.
If function definition, we have by default passed recurse=True. If it's True, it goes through all the files in the File path we have passed as well as going inside various folders inside the File Path recursively. Else if recurse=False, we just go through all files in the File Path we have passed without going inside various folders.
For the sake of understanding, let's take an example a .git directory, with the following file structure.
os.scandir returns an iterator of Directory objects. In Python os module, there is an os.listdir(path='.') which does the same functionality as scandir. Yet scandir gives a better performance for most of common use cases. [1]
It returns a list of file extensions as shown below with list comprehensions, where is_file() returns, if it's a file or whether it's pointing to a directory with followlinks.
If recurse=True, it goes through all the directories and works on files recursively. Let's look at the sources code and try to understand more.
Just to summarise how the get_files function is working it will be useful to look at the below illustration:
When recurse=False, for path bears. It returns just returns file README excluding (.gitignore) and directories.
While recurse=True, for path bears. It returns all valid files inside the root directory as well as in folders such grizzly, black, teddy, details, folder etc.
After that, it's passed to _get_files function, which returns the list of filenames to a list of pathlib Path of various filenames.
fs, the list of files returned. We are not passing files that are starting with . like .gitignore or .env as it's not usually very useful for our dataset to get as files. Also, it's not returning file extensions passed or f'.{f.split(".")[-1].lower()}' in extensions.
res on passing p/f for the list of files will become a list of paths as shown in the below result. Transforming from a list of file names, we are transforming it to a list of Pathlib module Path, pointing to various filenames.
I hope with this blog post, you now have understood how get_image_files, fetch the list of images under the hood by looking into the source code.
We are converting the path provided to us into a Pathlib object, and folders are converted to a special Python-like list called (L) based on the fastcore library. The extensions are converted to a set if it's being passed as a list, range, string etc. using setify. All the extensions are converted to lower case characters if any extension is in upper case. To read more about the setify function check .
I would highly recommend the functionality of os.walk by checking this . You can see that on iterating through os.walk(), we can get the directory path, and associate file path as a list. This is being passed to _get_files(p, f, extension) function.
This is how the get_image_files, returns a L object based on fastcore for any object. For the , the output of get_image_files and get_files is as following:
In case, if I have missed something or to provide feedback, please feel free to reach out to me .
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