In a Python3 CGI script, how can I read the raw form data in a POST?

Question

Yes, I'm aware of cgi.FieldStorage , but, being some form of unordered dictionary, it does not preserve the order of the original data (see below for proof). Since I wish to use this data with PayPal IPN, order is important PayPal docs here , which say "...you must send back the contents in the exact order they were received..."

Alternatively, os.environ.get('QUERY_STRING') looks perfect, however, it seems to only work for a GET . Example code: (myscript.py)

#!/usr/bin/python3

import cgi, os

query = os.environ.get('QUERY_STRING') or 'no query'
print ("Content-type: text/plain\n\n")
print("query=" + query)
form = cgi.FieldStorage()
for key in form.keys():
   print("\n" + key + '=' + form.getvalue(key))

Works with a GET from the browser, eg (note that foo is before ggg)

http://example.com/myscript.py/foo=bar&ggg=3&aaa=bbb&zzz=qqq

returns

query=foo=bar&ggg=3&aaa=bbb&zzz=qqq
ggg=3
foo=bar   << note that foo now comes after ggg
aaa=bbb
zzz=qqq

However, if I use Postman to POST

POST /myscript.py HTTP/1.1
Host: example.com
Cache-Control: no-cache
Content-Type: application/x-www-form-urlencoded

foo=bar&ggg=3&aaa=bbb&zzz=qqq

It returns no query string, and, as expected, the FormData did not preserve the order.

query=no query
ggg=3
foo=bar   << note that foo now comes after ggg
aaa=bbb
zzz=qqq

Answer 1

Not sure why nobody answered. After a little futzing I discovered that the solution is incredibly simple . Is that why nobody bothered to answer?

Just read from stdin: (this is Python3 code, not sure if Python2 would be any different)

query_string = sys.stdin.read()

There's one drawback: this is incompatible with cgi.FieldStorage() , since that will also try to read from stdin. So, if you also want a nice dictionary to look up query terms, there is one more simple step:

multiform = urllib.parse.parse_qs(query_string)

which, much like cgi.FieldStorage, returns a multimap , hence the name multiform.

For some more details, I blogged about it here .