how to send photo by telegram bot using multipart/form-data

Try this line of code

status = requests.post("https://api.telegram.org/bot<TOKEN>/sendPhoto?chat_id=" + data['chat_id'], files=files)

Both answers by Delimitry and Pyae Hlian Moe are correct in the sense that they work, but neither address the actual problem with the code you supplied.

The problem is that data is defined as:

data = {'chat_id', chat_id}

which is a set (not a dictionary) with two values: a string 'chat_id' and the content of chat_id, instead of

data = {'chat_id' : chat_id}

which is a dictionary with a key: the string 'chat_id' and a corresponding value stored in chat_id.

chat_id can be defined as part of the url, but similarly your original code should work as well - defining data and files as parameters for requests.post() - as long as both data and files variables are dictionaries.