Upload a File with Python

Since you said that your specific application is for use with the python cgi module, a quick google turns up plenty of examples. Here is the first one:

Minimal http upload cgi (Python recipe) (snip)

def save_uploaded_file (form_field, upload_dir):
    """This saves a file uploaded by an HTML form.
       The form_field is the name of the file input field from the form.
       For example, the following form_field would be "file_1":
           <input name="file_1" type="file">
       The upload_dir is the directory where the file will be written.
       If no file was uploaded or if the field does not exist then
       this does nothing.
    """
    form = cgi.FieldStorage()
    if not form.has_key(form_field): return
    fileitem = form[form_field]
    if not fileitem.file: return
    fout = file (os.path.join(upload_dir, fileitem.filename), 'wb')
    while 1:
        chunk = fileitem.file.read(100000)
        if not chunk: break
        fout.write (chunk)
    fout.close()

This code will grab the file input field, which will be a file-like object. Then it will read it, chunk by chunk, into an output file.

Update 04/12/15: Per comments, I have added in the updates to this old activestate snippet:

import shutil

def save_uploaded_file (form_field, upload_dir):
    form = cgi.FieldStorage()
    if not form.has_key(form_field): return
    fileitem = form[form_field]
    if not fileitem.file: return

    outpath = os.path.join(upload_dir, fileitem.filename)

    with open(outpath, 'wb') as fout:
        shutil.copyfileobj(fileitem.file, fout, 100000)

The web frame work Pyramid has a good example. http://docs.pylonsproject.org/projects/pyramid_cookbook/en/latest/forms/file_uploads.html

Here is my example code that I use with a working project.

    extension = os.path.splitext(request.POST[form_id_name].filename)[1]
    short_id = str(random.randint(1, 999999999))
    new_file_name =  short_id + extension
    input_file = request.POST[form_id_name].file
    file_path = os.path.join(os.environ['PROJECT_PATH'] + '/static/memberphotos/', new_file_name)

    output_file = open(file_path, 'wb')
    input_file.seek(0)
    while 1:
        data = input_file.read(2<<16)
        if not data:
            break
        output_file.write(data)
    output_file.close()