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In HTTP request, same query parameter with multiple values are allowed. For example, for query parameter brand, the query string looks like this: brand=foo&brand=bar. How to parse value of this parameter to a list in FastAPI? There are several different ways.

Using Type annotation and Query
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If you already know which parameter is going to have multiple values, you can use the following method (based on discussion here):

from fastapi import FastAPI, Request, Query

app = FastAPI()

@app.get('/multi_value')
def multi_value(brand: List[str] = Query(default=None)):
    return {"brand": brand}

In this way, the query parameter brand will be known by FastAPI to have a list of values. If you access the API like this: http://127.0.0.1:8000/multi_value?brand=foo&brand=bar, you can get correct response:

{"brand":["foo","bar"]}

Deal with the request directly
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If you do not know which parameter is going to have multiple values, you can also directly handle it with using the request object from FastAPI.

@app.get('/foo')
def foo(request: Request):
    query_params = request.query_params
    params_dict = {}
    for k in query_params.keys():
        params_dict[k] = query_params.getlist(k]
    return params_dict

If a query parameter can appear multiple time in the query, we shouldn’t use query_params.get() to get its values, because it will only return the last value. Similarly, query_params.items() and query_params.values() also only contain the last value for the query parameter.

Another way is to use parse_qs from urllib to parse the query string manually.

@app.get('/foo')
def foo(request: Request):
    # `request.url` is the complete URL for the HTTP request.
    # `request.url.query` contains only the query part from the whole URL. For example, if your complete
    # URL is http://127.0.0.1:8000/search2/foo?q=schuhe&color=blue&color=black, then the value for
    # request.url.query would be "q=schuhe&color=blue&color=black"
    params_dict = parse_qs(request.url.query, keep_blank_values=True)
    return params_dict

parse_qs() can handle parameter with several values correctly. It has a parameter keep_blank_values, if it is set to True, query parameter with no value will also be kept in the parsed result, and the corresponding value is set to an empty string.