How to Send Multipart Form Data with Files and Standard Form Data in Python?

How to Construct Multipart Form Data Requests in Python

Question: How can I send multipart form data in Python using the requests library? While I understand how to attach files, I'm struggling to incorporate standard form data into such requests.

Answer:

Requests automatically handles multipart form data when a files parameter is specified, resulting in a multipart/form-data POST request instead of an application/x-www-form-urlencoded POST.

Syntax:

from requests import post

response = post(
    url,
    files={
        "form_field_name": "form_field_value",     # No quotes needed for non-string values
    }
)

Example:

response = post("http://httpbin.org/post", files={"foo": "bar"})
assert response.status_code == 200

Advanced Control:

Use tuples to customize the filename, content type, and additional headers for each part. Tuple components include:

• Filename (optional)
• Content
• Content type (optional)
• Header dictionary (optional)

Example:

files = {"foo": (None, "bar")}   # No filename specified

Ordered Multiple Fields:

Use a list of tuples for ordered or multiple fields with the same name.

Handling Data and Files:

When using both data and files, a string data parameter will take precedence. Otherwise, both data and files are combined in the request.

Optional Libraries:

The requests-toolbelt project provides advanced Multipart support, allowing for:

• Streaming from file objects
• No default filename parameters
• Custom filename, part mime type, and extra headers using tuples

Example with requests-toolbelt:

import MultipartEncoder from requests_toolbelt

fields = {
    "foo": b"bar",    # Fields support bytes objects
    "spam": ("spam.txt", open("spam.txt", "rb"), "text/plain")   # Stream files
}

mp_encoder = MultipartEncoder(fields)

response = post(
    url,
    data=mp_encoder,
    headers={"Content-Type": mp_encoder.content_type}
)

Note: For the requests-toolbelt method, do not use files= argument as the MultipartEncoder is posted as the data payload.

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