Lib/cgi.py

#! /usr/local/bin/python

# NOTE: the above "/usr/local/bin/python" is NOT a mistake.  It is
# intentionally NOT "/usr/bin/env python".  On many systems
# (e.g. Solaris), /usr/local/bin is not in $PATH as passed to CGI
# scripts, and /usr/local/bin is the default directory where Python is
# installed, so /usr/bin/env would be unable to find python.  Granted,
# binary installations by Linux vendors often install Python in
# /usr/bin.  So let those vendors patch cgi.py to match their choice
# of installation.

"""Support module for CGI (Common Gateway Interface) scripts.

This module defines a number of utilities for use by CGI scripts
written in Python.
"""

# History
# -------
#
# Michael McLay started this module.  Steve Majewski changed the
# interface to SvFormContentDict and FormContentDict.  The multipart
# parsing was inspired by code submitted by Andreas Paepcke.  Guido van
# Rossum rewrote, reformatted and documented the module and is currently
# responsible for its maintenance.
#

__version__ = "2.6"


# Imports
# =======

from io import StringIO, BytesIO, TextIOWrapper
from collections.abc import Mapping
import sys
import os
import urllib.parse
from email.parser import FeedParser
from email.message import Message
import html
import locale
import tempfile

__all__ = ["MiniFieldStorage", "FieldStorage", "parse", "parse_multipart",
           "parse_header", "test", "print_exception", "print_environ",
           "print_form", "print_directory", "print_arguments",
           "print_environ_usage"]

# Logging support
# ===============

logfile = ""            # Filename to log to, if not empty
logfp = None            # File object to log to, if not None

def initlog(*allargs):
    """Write a log message, if there is a log file.

    Even though this function is called initlog(), you should always
    use log(); log is a variable that is set either to initlog
    (initially), to dolog (once the log file has been opened), or to
    nolog (when logging is disabled).

    The first argument is a format string; the remaining arguments (if
    any) are arguments to the % operator, so e.g.
        log("%s: %s", "a", "b")
    will write "a: b" to the log file, followed by a newline.

    If the global logfp is not None, it should be a file object to
    which log data is written.

    If the global logfp is None, the global logfile may be a string
    giving a filename to open, in append mode.  This file should be
    world writable!!!  If the file can't be opened, logging is
    silently disabled (since there is no safe place where we could
    send an error message).

    """
    global log, logfile, logfp
    if logfile and not logfp:
        try:
            logfp = open(logfile, "a")
        except OSError:
            pass
    if not logfp:
        log = nolog
    else:
        log = dolog
    log(*allargs)

def dolog(fmt, *args):
    """Write a log message to the log file.  See initlog() for docs."""
    logfp.write(fmt%args + "\n")

def nolog(*allargs):
    """Dummy function, assigned to log when logging is disabled."""
    pass

def closelog():
    """Close the log file."""
    global log, logfile, logfp
    logfile = ''
    if logfp:
        logfp.close()
        logfp = None
    log = initlog

log = initlog           # The current logging function


# Parsing functions
# =================

# Maximum input we will accept when REQUEST_METHOD is POST
# 0 ==> unlimited input
maxlen = 0

def parse(fp=None, environ=os.environ, keep_blank_values=0, strict_parsing=0,
          separators=('&', ';')):
    """Parse a query in the environment or from a file (default stdin)

        Arguments, all optional:

        fp              : file pointer; default: sys.stdin.buffer

        environ         : environment dictionary; default: os.environ

        keep_blank_values: flag indicating whether blank values in
            percent-encoded forms should be treated as blank strings.
            A true value indicates that blanks should be retained as
            blank strings.  The default false value indicates that
            blank values are to be ignored and treated as if they were
            not included.

        strict_parsing: flag indicating what to do with parsing errors.
            If false (the default), errors are silently ignored.
            If true, errors raise a ValueError exception.

        separators: valid separators in the query string, e.g. ('&',)
    """
    if fp is None:
        fp = sys.stdin

    # field keys and values (except for files) are returned as strings
    # an encoding is required to decode the bytes read from self.fp
    if hasattr(fp,'encoding'):
        encoding = fp.encoding
    else:
        encoding = 'latin-1'

    # fp.read() must return bytes
    if isinstance(fp, TextIOWrapper):
        fp = fp.buffer

    if not 'REQUEST_METHOD' in environ:
        environ['REQUEST_METHOD'] = 'GET'       # For testing stand-alone
    if environ['REQUEST_METHOD'] == 'POST':
        ctype, pdict = parse_header(environ['CONTENT_TYPE'])
        if ctype == 'multipart/form-data':
            return parse_multipart(fp, pdict, separators=separators)
        elif ctype == 'application/x-www-form-urlencoded':
            clength = int(environ['CONTENT_LENGTH'])
            if maxlen and clength > maxlen:
                raise ValueError('Maximum content length exceeded')
            qs = fp.read(clength).decode(encoding)
        else:
            qs = ''                     # Unknown content-type
        if 'QUERY_STRING' in environ:
            if qs: qs = qs + '&'
            qs = qs + environ['QUERY_STRING']
        elif sys.argv[1:]:
            if qs: qs = qs + '&'
            qs = qs + sys.argv[1]
        environ['QUERY_STRING'] = qs    # XXX Shouldn't, really
    elif 'QUERY_STRING' in environ:
        qs = environ['QUERY_STRING']
    else:
        if sys.argv[1:]:
            qs = sys.argv[1]
        else:
            qs = ""
        environ['QUERY_STRING'] = qs    # XXX Shouldn't, really
    return urllib.parse.parse_qs(qs, keep_blank_values, strict_parsing,
                                 encoding=encoding, separators=separators)


def parse_multipart(fp, pdict, encoding="utf-8", errors="replace",
                    separators=('&', ';')):
    """Parse multipart input.

    Arguments:
    fp   : input file
    pdict: dictionary containing other parameters of content-type header
    encoding, errors: request encoding and error handler, passed to
        FieldStorage

    Returns a dictionary just like parse_qs(): keys are the field names, each
    value is a list of values for that field. For non-file fields, the value
    is a list of strings.
    """
    # RFC 2046, Section 5.1 : The "multipart" boundary delimiters are always
    # represented as 7bit US-ASCII.
    boundary = pdict['boundary'].decode('ascii')
    ctype = "multipart/form-data; boundary={}".format(boundary)
    headers = Message()
    headers.set_type(ctype)
    try:
        headers['Content-Length'] = pdict['CONTENT-LENGTH']
    except KeyError:
        pass
    fs = FieldStorage(fp, headers=headers, encoding=encoding, errors=errors,
        environ={'REQUEST_METHOD': 'POST'}, separators=separators)
    return {k: fs.getlist(k) for k in fs}

def _parseparam(s):
    while s[:1] == ';':
        s = s[1:]
        end = s.find(';')
        while end > 0 and (s.count('"', 0, end) - s.count('\\"', 0, end)) % 2:
            end = s.find(';', end + 1)
        if end < 0:
            end = len(s)
        f = s[:end]
        yield f.strip()
        s = s[end:]

def parse_header(line):
    """Parse a Content-type like header.

    Return the main content-type and a dictionary of options.

    """
    parts = _parseparam(';' + line)
    key = parts.__next__()
    pdict = {}
    for p in parts:
        i = p.find('=')
        if i >= 0:
            name = p[:i].strip().lower()
            value = p[i+1:].strip()
            if len(value) >= 2 and value[0] == value[-1] == '"':
                value = value[1:-1]
                value = value.replace('\\\\', '\\').replace('\\"', '"')
            pdict[name] = value
    return key, pdict


# Classes for field storage
# =========================

class MiniFieldStorage:

    """Like FieldStorage, for use when no file uploads are possible."""

    # Dummy attributes
    filename = None
    list = None
    type = None
    file = None
    type_options = {}
    disposition = None
    disposition_options = {}
    headers = {}

    def __init__(self, name, value):
        """Constructor from field name and value."""
        self.name = name
        self.value = value
        # self.file = StringIO(value)

    def __repr__(self):
        """Return printable representation."""
        return "MiniFieldStorage(%r, %r)" % (self.name, self.value)


class FieldStorage:

    """Store a sequence of fields, reading multipart/form-data.

    This class provides naming, typing, files stored on disk, and
    more.  At the top level, it is accessible like a dictionary, whose
    keys are the field names.  (Note: None can occur as a field name.)
    The items are either a Python list (if there's multiple values) or
    another FieldStorage or MiniFieldStorage object.  If it's a single
    object, it has the following attributes:

    name: the field name, if specified; otherwise None

    filename: the filename, if specified; otherwise None; this is the
        client side filename, *not* the file name on which it is
        stored (that's a temporary file you don't deal with)

    value: the value as a *string*; for file uploads, this
        transparently reads the file every time you request the value
        and returns *bytes*

    file: the file(-like) object from which you can read the data *as
        bytes* ; None if the data is stored a simple string

    type: the content-type, or None if not specified

    type_options: dictionary of options specified on the content-type
        line

    disposition: content-disposition, or None if not specified

    disposition_options: dictionary of corresponding options

    headers: a dictionary(-like) object (sometimes email.message.Message or a
        subclass thereof) containing *all* headers

    The class is subclassable, mostly for the purpose of overriding
    the make_file() method, which is called internally to come up with
    a file open for reading and writing.  This makes it possible to
    override the default choice of storing all files in a temporary
    directory and unlinking them as soon as they have been opened.

    """
    def __init__(self, fp=None, headers=None, outerboundary=b'',
                 environ=os.environ, keep_blank_values=0, strict_parsing=0,
                 limit=None, encoding='utf-8', errors='replace',
                 max_num_fields=None, separators=('&', ';')):
        """Constructor.  Read multipart/* until last part.

        Arguments, all optional:

        fp              : file pointer; default: sys.stdin.buffer
            (not used when the request method is GET)
            Can be :
            1. a TextIOWrapper object
            2. an object whose read() and readline() methods return bytes

        headers         : header dictionary-like object; default:
            taken from environ as per CGI spec

        outerboundary   : terminating multipart boundary
            (for internal use only)

        environ         : environment dictionary; default: os.environ

        keep_blank_values: flag indicating whether blank values in
            percent-encoded forms should be treated as blank strings.
            A true value indicates that blanks should be retained as
            blank strings.  The default false value indicates that
            blank values are to be ignored and treated as if they were
            not included.

        strict_parsing: flag indicating what to do with parsing errors.
            If false (the default), errors are silently ignored.
            If true, errors raise a ValueError exception.

        limit : used internally to read parts of multipart/form-data forms,
            to exit from the reading loop when reached. It is the difference
            between the form content-length and the number of bytes already
            read

        encoding, errors : the encoding and error handler used to decode the
            binary stream to strings. Must be the same as the charset defined
            for the page sending the form (content-type : meta http-equiv or
            header)

        max_num_fields: int. If set, then __init__ throws a ValueError
            if there are more than n fields read by parse_qsl().

        separators: valid separators in the query string, e.g. ('&',).
            Passed directly to urllib.parse.parse_qsl internally.

        """
        method = 'GET'
        self.keep_blank_values = keep_blank_values
        self.strict_parsing = strict_parsing
        self.max_num_fields = max_num_fields
        self.separators = separators
        if 'REQUEST_METHOD' in environ:
            method = environ['REQUEST_METHOD'].upper()
        self.qs_on_post = None
        if method == 'GET' or method == 'HEAD':
            if 'QUERY_STRING' in environ:
                qs = environ['QUERY_STRING']
            elif sys.argv[1:]:
                qs = sys.argv[1]
            else:
                qs = ""
            qs = qs.encode(locale.getpreferredencoding(), 'surrogateescape')
            fp = BytesIO(qs)
            if headers is None:
                headers = {'content-type':
                           "application/x-www-form-urlencoded"}
        if headers is None:
            headers = {}
            if method == 'POST':
                # Set default content-type for POST to what's traditional
                headers['content-type'] = "application/x-www-form-urlencoded"
            if 'CONTENT_TYPE' in environ:
                headers['content-type'] = environ['CONTENT_TYPE']
            if 'QUERY_STRING' in environ:
                self.qs_on_post = environ['QUERY_STRING']
            if 'CONTENT_LENGTH' in environ:
                headers['content-length'] = environ['CONTENT_LENGTH']
        else:
            if not (isinstance(headers, (Mapping, Message))):
                raise TypeError("headers must be mapping or an instance of "
                                "email.message.Message")
        self.headers = headers
        if fp is None:
            self.fp = sys.stdin.buffer
        # self.fp.read() must return bytes
        elif isinstance(fp, TextIOWrapper):
            self.fp = fp.buffer
        else:
            if not (hasattr(fp, 'read') and hasattr(fp, 'readline')):
                raise TypeError("fp must be file pointer")
            self.fp = fp

        self.encoding = encoding
        self.errors = errors

        if not isinstance(outerboundary, bytes):
            raise TypeError('outerboundary must be bytes, not %s'
                            % type(outerboundary).__name__)
        self.outerboundary = outerboundary

        self.bytes_read = 0
        self.limit = limit

        # Process content-disposition header
        cdisp, pdict = "", {}
        if 'content-disposition' in self.headers:
            cdisp, pdict = parse_header(self.headers['content-disposition'])
        self.disposition = cdisp
        self.disposition_options = pdict
        self.name = None
        if 'name' in pdict:
            self.name = pdict['name']
        self.filename = None
        if 'filename' in pdict:
            self.filename = pdict['filename']
        self._binary_file = self.filename is not None

        # Process content-type header
        #
        # Honor any existing content-type header.  But if there is no
        # content-type header, use some sensible defaults.  Assume
        # outerboundary is "" at the outer level, but something non-false
        # inside a multi-part.  The default for an inner part is text/plain,
        # but for an outer part it should be urlencoded.  This should catch
        # bogus clients which erroneously forget to include a content-type
        # header.
        #
        # See below for what we do if there does exist a content-type header,
        # but it happens to be something we don't understand.
        if 'content-type' in self.headers:
            ctype, pdict = parse_header(self.headers['content-type'])
        elif self.outerboundary or method != 'POST':
            ctype, pdict = "text/plain", {}
        else:
            ctype, pdict = 'application/x-www-form-urlencoded', {}
        self.type = ctype
        self.type_options = pdict
        if 'boundary' in pdict:
            self.innerboundary = pdict['boundary'].encode(self.encoding,
                                                          self.errors)
        else:
            self.innerboundary = b""

        clen = -1
        if 'content-length' in self.headers:
            try:
                clen = int(self.headers['content-length'])
            except ValueError:
                pass
            if maxlen and clen > maxlen:
                raise ValueError('Maximum content length exceeded')
        self.length = clen
        if self.limit is None and clen >= 0:
            self.limit = clen

        self.list = self.file = None
        self.done = 0
        if ctype == 'application/x-www-form-urlencoded':
            self.read_urlencoded()
        elif ctype[:10] == 'multipart/':
            self.read_multi(environ, keep_blank_values, strict_parsing, separators)
        else:
            self.read_single()

    def __del__(self):
        try:
            self.file.close()
        except AttributeError:
            pass

    def __enter__(self):
        return self

    def __exit__(self, *args):
        self.file.close()

    def __repr__(self):
        """Return a printable representation."""
        return "FieldStorage(%r, %r, %r)" % (
                self.name, self.filename, self.value)

    def __iter__(self):
        return iter(self.keys())

    def __getattr__(self, name):
        if name != 'value':
            raise AttributeError(name)
        if self.file:
            self.file.seek(0)
            value = self.file.read()
            self.file.seek(0)
        elif self.list is not None:
            value = self.list
        else:
            value = None
        return value

    def __getitem__(self, key):
        """Dictionary style indexing."""
        if self.list is None:
            raise TypeError("not indexable")
        found = []
        for item in self.list:
            if item.name == key: found.append(item)
        if not found:
            raise KeyError(key)
        if len(found) == 1:
            return found[0]
        else:
            return found

    def getvalue(self, key, default=None):
        """Dictionary style get() method, including 'value' lookup."""
        if key in self:
            value = self[key]
            if isinstance(value, list):
                return [x.value for x in value]
            else:
                return value.value
        else:
            return default

    def getfirst(self, key, default=None):
        """ Return the first value received."""
        if key in self:
            value = self[key]
            if isinstance(value, list):
                return value[0].value
            else:
                return value.value
        else:
            return default

    def getlist(self, key):
        """ Return list of received values."""
        if key in self:
            value = self[key]
            if isinstance(value, list):
                return [x.value for x in value]
            else:
                return [value.value]
        else:
            return []

    def keys(self):
        """Dictionary style keys() method."""
        if self.list is None:
            raise TypeError("not indexable")
        return list(set(item.name for item in self.list))

    def __contains__(self, key):
        """Dictionary style __contains__ method."""
        if self.list is None:
            raise TypeError("not indexable")
        return any(item.name == key for item in self.list)

    def __len__(self):
        """Dictionary style len(x) support."""
        return len(self.keys())

    def __bool__(self):
        if self.list is None:
            raise TypeError("Cannot be converted to bool.")
        return bool(self.list)

    def read_urlencoded(self):
        """Internal: read data in query string format."""
        qs = self.fp.read(self.length)
        if not isinstance(qs, bytes):
            raise ValueError("%s should return bytes, got %s" \
                             % (self.fp, type(qs).__name__))
        qs = qs.decode(self.encoding, self.errors)
        if self.qs_on_post:
            qs += '&' + self.qs_on_post
        query = urllib.parse.parse_qsl(
            qs, self.keep_blank_values, self.strict_parsing,
            encoding=self.encoding, errors=self.errors,
            max_num_fields=self.max_num_fields,
            separators=self.separators)
        self.list = [MiniFieldStorage(key, value) for key, value in query]
        self.skip_lines()

    FieldStorageClass = None

    def read_multi(self, environ, keep_blank_values, strict_parsing,
                   separators=('&', ';')):
        """Internal: read a part that is itself multipart."""
        ib = self.innerboundary
        if not valid_boundary(ib):
            raise ValueError('Invalid boundary in multipart form: %r' % (ib,))
        self.list = []
        if self.qs_on_post:
            query = urllib.parse.parse_qsl(
                self.qs_on_post, self.keep_blank_values, self.strict_parsing,
                encoding=self.encoding, errors=self.errors,
                max_num_fields=self.max_num_fields,
                separators=self.separators)
            self.list.extend(MiniFieldStorage(key, value) for key, value in query)

        klass = self.FieldStorageClass or self.__class__
        first_line = self.fp.readline() # bytes
        if not isinstance(first_line, bytes):
            raise ValueError("%s should return bytes, got %s" \
                             % (self.fp, type(first_line).__name__))
        self.bytes_read += len(first_line)

        # Ensure that we consume the file until we've hit our inner boundary
        while (first_line.strip() != (b"--" + self.innerboundary) and
                first_line):
            first_line = self.fp.readline()
            self.bytes_read += len(first_line)

        # Propagate max_num_fields into the sub class appropriately
        max_num_fields = self.max_num_fields
        if max_num_fields is not None:
            max_num_fields -= len(self.list)

        while True:
            parser = FeedParser()
            hdr_text = b""
            while True:
                data = self.fp.readline()
                hdr_text += data
                if not data.strip():
                    break
            if not hdr_text:
                break
            # parser takes strings, not bytes
            self.bytes_read += len(hdr_text)
            parser.feed(hdr_text.decode(self.encoding, self.errors))
            headers = parser.close()

            # Some clients add Content-Length for part headers, ignore them
            if 'content-length' in headers:
                del headers['content-length']

            limit = None if self.limit is None \
                else self.limit - self.bytes_read
            part = klass(self.fp, headers, ib, environ, keep_blank_values,
                         strict_parsing, limit,
                         self.encoding, self.errors, max_num_fields,
                         separators)

            if max_num_fields is not None:
                max_num_fields -= 1
                if part.list:
                    max_num_fields -= len(part.list)
                if max_num_fields < 0:
                    raise ValueError('Max number of fields exceeded')

            self.bytes_read += part.bytes_read
            self.list.append(part)
            if part.done or self.bytes_read >= self.length > 0:
                break
        self.skip_lines()

    def read_single(self):
        """Internal: read an atomic part."""
        if self.length >= 0:
            self.read_binary()
            self.skip_lines()
        else:
            self.read_lines()
        self.file.seek(0)

    bufsize = 8*1024            # I/O buffering size for copy to file

    def read_binary(self):
        """Internal: read binary data."""
        self.file = self.make_file()
        todo = self.length
        if todo >= 0:
            while todo > 0:
                data = self.fp.read(min(todo, self.bufsize)) # bytes
                if not isinstance(data, bytes):
                    raise ValueError("%s should return bytes, got %s"
                                     % (self.fp, type(data).__name__))
                self.bytes_read += len(data)
                if not data:
                    self.done = -1
                    break
                self.file.write(data)
                todo = todo - len(data)

    def read_lines(self):
        """Internal: read lines until EOF or outerboundary."""
        if self._binary_file:
            self.file = self.__file = BytesIO() # store data as bytes for files
        else:
            self.file = self.__file = StringIO() # as strings for other fields
        if self.outerboundary:
            self.read_lines_to_outerboundary()
        else:
            self.read_lines_to_eof()

    def __write(self, line):
        """line is always bytes, not string"""
        if self.__file is not None:
            if self.__file.tell() + len(line) > 1000:
                self.file = self.make_file()
                data = self.__file.getvalue()
                self.file.write(data)
                self.__file = None
        if self._binary_file:
            # keep bytes
            self.file.write(line)
        else:
            # decode to string
            self.file.write(line.decode(self.encoding, self.errors))

    def read_lines_to_eof(self):
        """Internal: read lines until EOF."""
        while 1:
            line = self.fp.readline(1<<16) # bytes
            self.bytes_read += len(line)
            if not line:
                self.done = -1
                break
            self.__write(line)

    def read_lines_to_outerboundary(self):
        """Internal: read lines until outerboundary.
        Data is read as bytes: boundaries and line ends must be converted
        to bytes for comparisons.
        """
        next_boundary = b"--" + self.outerboundary
        last_boundary = next_boundary + b"--"
        delim = b""
        last_line_lfend = True
        _read = 0
        while 1:

            if self.limit is not None and 0 <= self.limit <= _read:
                break
            line = self.fp.readline(1<<16) # bytes
            self.bytes_read += len(line)
            _read += len(line)
            if not line:
                self.done = -1
                break
            if delim == b"\r":
                line = delim + line
                delim = b""
            if line.startswith(b"--") and last_line_lfend:
                strippedline = line.rstrip()
                if strippedline == next_boundary:
                    break
                if strippedline == last_boundary:
                    self.done = 1
                    break
            odelim = delim
            if line.endswith(b"\r\n"):
                delim = b"\r\n"
                line = line[:-2]
                last_line_lfend = True
            elif line.endswith(b"\n"):
                delim = b"\n"
                line = line[:-1]
                last_line_lfend = True
            elif line.endswith(b"\r"):
                # We may interrupt \r\n sequences if they span the 2**16
                # byte boundary
                delim = b"\r"
                line = line[:-1]
                last_line_lfend = False
            else:
                delim = b""
                last_line_lfend = False
            self.__write(odelim + line)

    def skip_lines(self):
        """Internal: skip lines until outer boundary if defined."""
        if not self.outerboundary or self.done:
            return
        next_boundary = b"--" + self.outerboundary
        last_boundary = next_boundary + b"--"
        last_line_lfend = True
        while True:
            line = self.fp.readline(1<<16)
            self.bytes_read += len(line)
            if not line:
                self.done = -1
                break
            if line.endswith(b"--") and last_line_lfend:
                strippedline = line.strip()
                if strippedline == next_boundary:
                    break
                if strippedline == last_boundary:
                    self.done = 1
                    break
            last_line_lfend = line.endswith(b'\n')

    def make_file(self):
        """Overridable: return a readable & writable file.

        The file will be used as follows:
        - data is written to it
        - seek(0)
        - data is read from it

        The file is opened in binary mode for files, in text mode
        for other fields

        This version opens a temporary file for reading and writing,
        and immediately deletes (unlinks) it.  The trick (on Unix!) is
        that the file can still be used, but it can't be opened by
        another process, and it will automatically be deleted when it
        is closed or when the current process terminates.

        If you want a more permanent file, you derive a class which
        overrides this method.  If you want a visible temporary file
        that is nevertheless automatically deleted when the script
        terminates, try defining a __del__ method in a derived class
        which unlinks the temporary files you have created.

        """
        if self._binary_file:
            return tempfile.TemporaryFile("wb+")
        else:
            return tempfile.TemporaryFile("w+",
                encoding=self.encoding, newline = '\n')


# Test/debug code
# ===============

def test(environ=os.environ):
    """Robust test CGI script, usable as main program.

    Write minimal HTTP headers and dump all information provided to
    the script in HTML form.

    """
    print("Content-type: text/html")
    print()
    sys.stderr = sys.stdout
    try:
        form = FieldStorage()   # Replace with other classes to test those
        print_directory()
        print_arguments()
        print_form(form)
        print_environ(environ)
        print_environ_usage()
        def f():
            exec("testing print_exception() -- <I>italics?</I>")
        def g(f=f):
            f()
        print("<H3>What follows is a test, not an actual exception:</H3>")
        g()
    except:
        print_exception()

    print("<H1>Second try with a small maxlen...</H1>")

    global maxlen
    maxlen = 50
    try:
        form = FieldStorage()   # Replace with other classes to test those
        print_directory()
        print_arguments()
        print_form(form)
        print_environ(environ)
    except:
        print_exception()

def print_exception(type=None, value=None, tb=None, limit=None):
    if type is None:
        type, value, tb = sys.exc_info()
    import traceback
    print()
    print("<H3>Traceback (most recent call last):</H3>")
    list = traceback.format_tb(tb, limit) + \
           traceback.format_exception_only(type, value)
    print("<PRE>%s<B>%s</B></PRE>" % (
        html.escape("".join(list[:-1])),
        html.escape(list[-1]),
        ))
    del tb

def print_environ(environ=os.environ):
    """Dump the shell environment as HTML."""
    keys = sorted(environ.keys())
    print()
    print("<H3>Shell Environment:</H3>")
    print("<DL>")
    for key in keys:
        print("<DT>", html.escape(key), "<DD>", html.escape(environ[key]))
    print("</DL>")
    print()

def print_form(form):
    """Dump the contents of a form as HTML."""
    keys = sorted(form.keys())
    print()
    print("<H3>Form Contents:</H3>")
    if not keys:
        print("<P>No form fields.")
    print("<DL>")
    for key in keys:
        print("<DT>" + html.escape(key) + ":", end=' ')
        value = form[key]
        print("<i>" + html.escape(repr(type(value))) + "</i>")
        print("<DD>" + html.escape(repr(value)))
    print("</DL>")
    print()

def print_directory():
    """Dump the current directory as HTML."""
    print()
    print("<H3>Current Working Directory:</H3>")
    try:
        pwd = os.getcwd()
    except OSError as msg:
        print("OSError:", html.escape(str(msg)))
    else:
        print(html.escape(pwd))
    print()

def print_arguments():
    print()
    print("<H3>Command Line Arguments:</H3>")
    print()
    print(sys.argv)
    print()

def print_environ_usage():
    """Dump a list of environment variables used by CGI as HTML."""
    print("""
<H3>These environment variables could have been set:</H3>
<UL>
<LI>AUTH_TYPE
<LI>CONTENT_LENGTH
<LI>CONTENT_TYPE
<LI>DATE_GMT
<LI>DATE_LOCAL
<LI>DOCUMENT_NAME
<LI>DOCUMENT_ROOT
<LI>DOCUMENT_URI
<LI>GATEWAY_INTERFACE
<LI>LAST_MODIFIED
<LI>PATH
<LI>PATH_INFO
<LI>PATH_TRANSLATED
<LI>QUERY_STRING
<LI>REMOTE_ADDR
<LI>REMOTE_HOST
<LI>REMOTE_IDENT
<LI>REMOTE_USER
<LI>REQUEST_METHOD
<LI>SCRIPT_NAME
<LI>SERVER_NAME
<LI>SERVER_PORT
<LI>SERVER_PROTOCOL
<LI>SERVER_ROOT
<LI>SERVER_SOFTWARE
</UL>
In addition, HTTP headers sent by the server may be passed in the
environment as well.  Here are some common variable names:
<UL>
<LI>HTTP_ACCEPT
<LI>HTTP_CONNECTION
<LI>HTTP_HOST
<LI>HTTP_PRAGMA
<LI>HTTP_REFERER
<LI>HTTP_USER_AGENT
</UL>
""")


# Utilities
# =========

def valid_boundary(s):
    import re
    if isinstance(s, bytes):
        _vb_pattern = b"^[ -~]{0,200}[!-~]$"
    else:
        _vb_pattern = "^[ -~]{0,200}[!-~]$"
    return re.match(_vb_pattern, s)

# Invoke mainline
# ===============

# Call test() when this file is run as a script (not imported as a module)
if __name__ == '__main__':
    test()