python simple wsgi file upload script - What is wrong?

Question

import os, cgi

#self_hosting script
tags = """<form enctype="multipart/form-data" action="save_file.py" method="post">
<p>File: <input type="file" name="file"></p>
<p><input type="submit" value="Upload"></p>
</form>"""

def Request(environ, start_response):
    # use cgi module to read data
    form = cgi.FieldStorage(fp=environ['wsgi.input'], environ=environ, keep_blank_values=True)

    try:
        fileitem = form['file']
    except KeyError:
        fileitem = None

    if fileitem and fileitem.file:
        fn = os.path.basename(fileitem.filename)
        with open(fn, 'wb') as f:
            data = fileitem.file.read(1024)
            while data:
                f.write(data)
                data = fileitem.file.read(1024)

            message = 'The file "' + fn + '" was uploaded successfully'

    else :
        message = 'please upload a file.'


    start_response('200 OK', [('Content-type','text/html')])

    return [message + "<br / >" + tags]

Above is my python wsgi script that receives a file and writes it to the disk. However, upon executing (with a file selected):

Internal Server Error An error occurred processing this request. Request handler failed

Traceback (most recent call last):
  File "C:\Python26\Http\Isapi.py", line 110, in Request
    return Handler(Name)
  File "C:\Python26\Http\Isapi.py", line 93, in 
    "/apps/py/" : lambda P: RunWSGIWrapper(P),
  File "C:\Python26\Http\Isapi.py", line 86, in RunWSGIWrapper
    return RunWSGI(ScriptHandlers[Path])
  File "C:\Python26\Http\WSGI.py", line 155, in RunWSGI
    Result = Application(Environ, StartResponse)
  File "\\?\C:\Python26\html\save_file.py", line 13, in Request
    form = cgi.FieldStorage(fp=environ['wsgi.input'], environ=environ, keep_blank_values=True)
  File "C:\Python26\Lib\cgi.py", line 496, in __init__
    self.read_multi(environ, keep_blank_values, strict_parsing)
  File "C:\Python26\Lib\cgi.py", line 620, in read_multi
    environ, keep_blank_values, strict_parsing)
  File "C:\Python26\Lib\cgi.py", line 498, in __init__
    self.read_single()
  File "C:\Python26\Lib\cgi.py", line 635, in read_single
    self.read_lines()
  File "C:\Python26\Lib\cgi.py", line 657, in read_lines
    self.read_lines_to_outerboundary()
  File "C:\Python26\Lib\cgi.py", line 685, in read_lines_to_outerboundary
    line = self.fp.readline(1<<16)
AttributeError: 'module' object has no attribute 'readline'

Being pretty daft at wsgi and cgi module I have no idea to progress at this moment. any clues?

Answer 1

environ['wsgi.input'] is a stream like object. You need to cache it firstly to file like object, eg: tempfile.TemporaryFile or StringIO (io.BytesIO in python3):

from tempfile import TemporaryFile
import os, cgi

def read(environ):
    length = int(environ.get('CONTENT_LENGTH', 0))
    stream = environ['wsgi.input']
    body = TemporaryFile(mode='w+b')
    while length > 0:
        part = stream.read(min(length, 1024*200)) # 200KB buffer size
        if not part: break
        body.write(part)
        length -= len(part)
    body.seek(0)
    environ['wsgi.input'] = body
    return body

def Request(environ, start_response):
    # use cgi module to read data
    body = read(environ)
    form = cgi.FieldStorage(fp=body, environ=environ, keep_blank_values=True)
    # rest of your code

For safety reason consider to mask environ value which you pass to FieldStorage

Answer 2

Self - answer : Sorry I didn't said that This was PyISAPIe-specific problem. the file-like object environ['wsgi.input'] does not have readline() method like other environ varibles in different wsgi implementations would do.

the (inefficient workaround) is saving everything from environ['wsgi.input'] into a tempfile and pass it to FieldStorage.

So :

import tempfile, cgi
def some_wsgi_app(environ, start_response):
    temp_file = tempfile.TemporaryFile()
    temp_file.write(environ['wsgi.input'].read()) # or use buffered read()
    temp_file.seek(0)
    form = cgi.FieldStorage(fp=temp_file, environ=environ, keep_blank_values=True)
    # do_something #
    temp_file.close() #close and destroy temp file

    # ... start_response, return ... #

However above example will fail to operate properly if uploaded data from user is too big.