How to handle comma-separated list query parameters in FastAPI?

Detailed explanation of the comma-separated processing method of FastAPI list parameters

When building APIs using FastAPI, it is often necessary to process list-type query parameters. FastAPI parses query parameters with the same name into a list by default. For example, ?source=manual&source=vdna will be parsed as source=['manual', 'vdna'] . But sometimes we need to pass list parameters with comma separated strings, such as ?source=manual,vdna . This article introduces two solutions.

The root of the problem: In the original code, source parameter was defined as list[source] type, causing FastAPI to parse duplicate parameters into lists instead of comma-separated strings.

Method 1: String parameters and manual parsing

The most direct way is to change the source parameter type to Optional[str] and manually parse comma-separated strings inside the function. This method is simple and easy to understand without additional dependencies.

Code example:

 @review.get('/list', summary='get list to be reviewed')
def list_await_review(
    # ... other parameters ...
    source: Optional[str] = Query(None, description="Please use `,` to split multiple parameters, such as `Manual,vDNA, text-match`",
    # ... other parameters ...
):
    parsed_source = source.split(",") if source else []
    # ...The subsequent code uses parsed_source...

This method parses source into a string and then splits it into a list using split(",") method. If source is empty, parsed_source is an empty list.

Method 2: Custom parsing process (advanced method)

A more flexible approach is to customize the parsing process, such as creating custom dependencies to handle comma-separated strings. This approach can handle more complex parsing logic, but requires more code writing and a deeper understanding of FastAPI's dependency injection mechanism. The official document recommends using Annotated enhanced parameter checksum type prompts, which is more advantageous when dealing with complex parameters, but the implementation details are beyond the scope of this article.

Which method to choose depends on your needs and FastAPI proficiency. For simple comma-separated list parameters, Method 1 is efficient and concise enough; for more complex scenarios, Method 2 provides greater flexibility.

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