Confusing use of object_name in example for S3 upload using presigned URL

[dennisvang]

Problem

Please consider the following relevant parts of the example from generating-a-presigned-url-to-upload-a-file:

# Generate a presigned S3 POST URL
object_name = 'OBJECT_NAME'
response = create_presigned_post('BUCKET_NAME', object_name)

As far as I know (see e.g. botocore source), this hardcodes the object name 'OBJECT_NAME' into the pre-signed post's policy string.

As a result, the filename that is supplied in the subsequent part of the example, i.e. the {'file': (object_name, f)} in the code below, is ignored. In other words, we could say {'file': ('something completely different', f)}, but the object in our bucket would still be named 'OBJECT_NAME'.

# Demonstrate how another Python program can use the presigned URL to upload a file
with open(object_name, 'rb') as f:
    files = {'file': (object_name, f)}
    http_response = requests.post(response['url'], data=response['fields'], files=files)

This is even more confusing if you consider the HTML form example that is provided at the bottom of the documentation page, which uses an <input type="file"> to select a file, but the object name is already fixed, so the name of the selected file will be ignored.

...
      <input type="hidden" name="policy" value="VALUE" />
      <input type="hidden" name="signature" value="VALUE" />
    File:
      <input type="file"   name="file" /> <br />
      <input type="submit" name="submit" value="Upload to Amazon S3" />
...

The result is a catch-22:

1. we need an object_name in order to generate the pre-signed post
2. the pre-signed post data is needed to build the HTML form
3. the HTML form allows us to select a file
4. the name of the selected file should (typically) be part of the object_name, but that was already fixed in step 1

Proposed solution

As can be seen in the botocore source, we can

... optionally add ${filename} to the end to attach the submitted filename. ...

Perhaps it would be better to modify the example along these lines:

Specify a variable filename by adding ${filename} to the object_name, as in

# Generate a presigned S3 POST URL
object_name = 'my/path/${filename}'
response = create_presigned_post('BUCKET_NAME', object_name)

then specify a separate file name, to make the distinction clear

# Demonstrate how another Python program can use the presigned URL to upload a file
filename = 'my_file'
with open(filename, 'rb') as f:
    files = {'file': (filename, f)}
    http_response = requests.post(response['url'], data=response['fields'], files=files)

The HTML code can then remain unaltered, as the filename from the file input will be used properly.

Links

https://boto3.amazonaws.com/v1/documentation/api/latest/guide/s3-presigned-urls.html#generating-a-presigned-url-to-upload-a-file

[tim-finnigan]

Thanks for the suggestion, there may be an opportunity to improve the documentation here. We can leave this open to track for now and get input from others.