Testing applications that use fsspec.open("s3://...") #1785

dionhaefner · 2025-01-30T11:35:31Z

dionhaefner
Jan 30, 2025

I'm trying to test an application that uses fsspec under the hood to open files passed in by users as s3://... strings. I thought I could use the moto proxy mode for this, but it doesn't seem to work (looks like fsspec is trying to hit live AWS instead, with a permissions error).

Helpful for any pointers on how to test S3 integration effectively.

Here's what I tried:

from moto import mock_aws
import json
import os

os.environ["TEST_PROXY_MODE"] = "true"

with mock_aws():
    import boto3
    import fsspec

    client = boto3.client("s3")
    client.create_bucket(Bucket="test-bucket")

    client.put_object(
        Bucket="test-bucket",
        Key="foo.json",
        Body=json.dumps({"inputs": "foo"}),
    )

    with fsspec.open("s3://test-bucket/foo.json") as f:
        print(f.read())

$ moto_proxy &
$ python foo.py
INFO 2025-01-30 12:31:34,189 - POST http://localhost:5005/moto-api/reset
/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/urllib3/connectionpool.py:1097: InsecureRequestWarning: Unverified HTTPS request is being made to host 'localhost'. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#tls-warnings
  warnings.warn(
INFO 2025-01-30 12:31:34,329 - PUT https://test-bucket.s3.amazonaws.com/
/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/urllib3/connectionpool.py:1097: InsecureRequestWarning: Unverified HTTPS request is being made to host 'localhost'. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#tls-warnings
  warnings.warn(
INFO 2025-01-30 12:31:34,404 - PUT https://test-bucket.s3.amazonaws.com/foo.json
Traceback (most recent call last):
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 114, in _error_wrapper
    return await func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/aiobotocore/client.py", line 412, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (403) when calling the HeadObject operation: Forbidden

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/dion/codes/tesseract/foo.py", line 20, in <module>
    with fsspec.open("s3://test-bucket/foo.json") as f:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/core.py", line 105, in __enter__
    f = self.fs.open(self.path, mode=mode)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/spec.py", line 1310, in open
    f = self._open(
        ^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 699, in _open
    return S3File(
           ^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 2238, in __init__
    super().__init__(
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/spec.py", line 1884, in __init__
    self.size = self.details["size"]
                ^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/spec.py", line 1897, in details
    self._details = self.fs.info(self.path)
                    ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/asyn.py", line 118, in wrapper
    return sync(self.loop, func, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/asyn.py", line 103, in sync
    raise return_result
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/asyn.py", line 56, in _runner
    result[0] = await coro
                ^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 1426, in _info
    out = await self._call_s3(
          ^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 371, in _call_s3
    return await _error_wrapper(
           ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 146, in _error_wrapper
    raise err
PermissionError: Forbidden

When not using proxy mode, I get this traceback instead:

Traceback (most recent call last):
  File "/Users/dion/codes/tesseract/foo.py", line 20, in <module>
    with fsspec.open("s3://test-bucket/foo.json") as f:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/core.py", line 105, in __enter__
    f = self.fs.open(self.path, mode=mode)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/spec.py", line 1310, in open
    f = self._open(
        ^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 699, in _open
    return S3File(
           ^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 2238, in __init__
    super().__init__(
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/spec.py", line 1884, in __init__
    self.size = self.details["size"]
                ^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/spec.py", line 1897, in details
    self._details = self.fs.info(self.path)
                    ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/asyn.py", line 118, in wrapper
    return sync(self.loop, func, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/asyn.py", line 103, in sync
    raise return_result
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/fsspec/asyn.py", line 56, in _runner
    result[0] = await coro
                ^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 1426, in _info
    out = await self._call_s3(
          ^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 371, in _call_s3
    return await _error_wrapper(
           ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 146, in _error_wrapper
    raise err
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/s3fs/core.py", line 114, in _error_wrapper
    return await func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/aiobotocore/client.py", line 394, in _make_api_call
    http, parsed_response = await self._make_request(
                            ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/aiobotocore/client.py", line 420, in _make_request
    return await self._endpoint.make_request(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/aiobotocore/endpoint.py", line 97, in _send_request
    success_response, exception = await self._get_response(
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/aiobotocore/endpoint.py", line 139, in _get_response
    success_response, exception = await self._do_get_response(
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/aiobotocore/endpoint.py", line 191, in _do_get_response
    response_dict = await convert_to_response_dict(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dion/.virtualenvs/science/lib/python3.12/site-packages/aiobotocore/endpoint.py", line 49, in convert_to_response_dict
    for k, v in http_response.raw.raw_headers
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'MockRawResponse' object has no attribute 'raw_headers'

Answered by dionhaefner

Jan 30, 2025

Example above is using mock_aws, which raises both with and without proxy mode (different errors tho).

Thanks for the pointer regarding global config though, that's helpful! This works now, even without monkeypatching anything:

import os
port = 5123
server_url = f"http://localhost:{port}"
os.environ["FSSPEC_S3_ENDPOINT_URL"] = server_url

import json
import boto3
import fsspec
from moto.server import ThreadedMotoServer

server = ThreadedMotoServer(port=port)
server.start()

client = boto3.client("s3", endpoint_url=server_url)
client.create_bucket(Bucket="test-bucket")

client.put_object(
    Bucket="test-bucket",
    Key="foo.json",
    Body=json.dumps({"inputs": "foo"}),
)

with fsspec.open

View full answer

martindurant · 2025-01-30T14:13:25Z

martindurant
Jan 30, 2025
Maintainer

The above LLM answer is essentially correct, but the key is the argument endpoint_url - you use this to tell s3fs which S3 to talk to.

Where did you encounter the environment variable TEST_PROXY_MODE, is this documented?

5 replies

dionhaefner Jan 30, 2025
Author

Believe it or not, this is actually not an LLM answer, just something I wrote myself 😉

the key is the argument endpoint_url - you use this to tell s3fs which S3 to talk to.

I don't have control over the fsspec.open call from the API I'm trying to test. I was hoping I could use moto proxy mode to intercept the calls to S3 and replace them with their mocked equivalent.

Where did you encounter the environment variable TEST_PROXY_MODE, is this documented?

It's from moto's proxy mode:

https://docs.getmoto.org/en/latest/docs/proxy_mode.html#python-configuration

martindurant Jan 30, 2025
Maintainer

It's from moto's proxy mode:

OK, then you will need to use mock_aws as a decorator or context manager. This isn't quite what the s3fs test suite does (it wants to be more explicit and control where moto runs), but I expect ti to work.

Alternatively, fsspec supports configuration allowing you to pass default arguments to filesystem instantiation. https://filesystem-spec.readthedocs.io/en/latest/features.html#configuration
A file in “~/.config/fsspec/conf.json” with {"s3": {"endpoint_url": "..."}} or FSSPEC_S3_ENDPOINT_URL=... should work.

martindurant Jan 30, 2025
Maintainer

Believe it or not, this is actually not an LLM answer, just something I wrote myself

Apologies; maybe we are all a bit LLM now.

dionhaefner Jan 30, 2025
Author

Example above is using mock_aws, which raises both with and without proxy mode (different errors tho).

Thanks for the pointer regarding global config though, that's helpful! This works now, even without monkeypatching anything:

import os
port = 5123
server_url = f"http://localhost:{port}"
os.environ["FSSPEC_S3_ENDPOINT_URL"] = server_url

import json
import boto3
import fsspec
from moto.server import ThreadedMotoServer

server = ThreadedMotoServer(port=port)
server.start()

client = boto3.client("s3", endpoint_url=server_url)
client.create_bucket(Bucket="test-bucket")

client.put_object(
    Bucket="test-bucket",
    Key="foo.json",
    Body=json.dumps({"inputs": "foo"}),
)

with fsspec.open("s3://test-bucket/foo.json") as f:
    print(f.read())

Answer selected by dionhaefner

martindurant Jan 30, 2025
Maintainer

Just to note that this:

AttributeError: 'MockRawResponse' object has no attribute 'raw_headers'

looks like it might be a version mismatch between moto and botocore, but I'm not sure.

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