Transcript Using Python for CGI programming

Using Python for CGI
programming
Guido van Rossum
CNRI
(Corporation for National Research Initiatives, Reston, Virginia, USA)
[email protected]
www.python.org
11/12/1999
© 1999 CNRI, Guido van Rossum
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Overview
• 1 minute advocacy
• 30 minutes basic Python tutorial
• 30 minutes on Python CGI programming
• 30 minutes CGI case study: FAQ wizard
• Spanish Inquisition
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Why Python?
• Have your cake and eat it, too:
Productivity and readable code
• VHLLs will gain on system languages
(John Ousterhout)
• "Life is better without braces"
(Bruce Eckel)
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Basic Python tutorial
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Tutorial outline
• interactive "shell"
• basic types: numbers, strings
• container types: lists, dictionaries, tuples
• variables
• control structures
• functions & procedures
• classes & instances
• modules & packages
• exceptions
• files & standard library
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Interactive “shell”
• Great for learning the language
• Great for experimenting with the library
• Great for testing your own modules
• Type statements or expressions at prompt:
>>> print "Hello, world"
Hello, world
>>> x = 12**2
>>> x/2
72
>>> # this is a comment
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Numbers
• The usual suspects
• 12, 3.14, 0xFF, 0377, (-1+2)*3/4**5, abs(x), 0<x<=5
• C-style shifting & masking
• 1<<16, x&0xff, x|1, ~x, x^y
• Integer division truncates :-(
• 1/2 -> 0
# float(1)/2 -> 0.5
• Long (arbitrary precision), complex
• 2L**100 -> 1267650600228229401496703205376L
• 1j**2 -> (-1+0j)
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Strings
• "hello"+"world"
"helloworld"
• "hello"*3
"hellohellohello" # repetition
• "hello"[0]
"h"
# indexing
• "hello"[-1]
"o"
# (from end)
• "hello"[1:4]
"ell"
# slicing
• len("hello")
5
# size
• "hello" < "jello"
1
# comparison
• "e" in "hello" 1
# concatenation
# search
• "escapes: \n etc, \033 etc, \xff etc"
• 'single quotes' '''triple quotes''' r"raw strings"
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Lists
• a = [99, "bottles of beer", ["on", "the", "wall"]]
• Flexible arrays, not Lisp-like linked lists
• Same operators as for strings
• a+b, a*3, a[0], a[-1], a[1:], len(a)
• Item and slice assignment
• a[0] = 98
• a[1:2] = ["bottles", "of", "beer"]
-> [98, "bottles", "of", "beer", ["on", "the", "wall"]]
• del a[-1]
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# -> [98, "bottles", "of", "beer"]
© 1999 CNRI, Guido van Rossum
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More list operations
>>> a = range(5)
# [0,1,2,3,4]
>>> a.append(5)
# [0,1,2,3,4,5]
>>> a.pop()
# [0,1,2,3,4]
5
>>> a.insert(0, 5.5)
# [5.5,0,1,2,3,4]
>>> a.pop(0)
# [0,1,2,3,4]
5.5
>>> a.reverse()
# [4,3,2,1,0]
>>> a.sort()
# [0,1,2,3,4]
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Dictionaries
• Hash tables, "associative arrays"
• d = {"duck": "eend", "water": "water"}
• Lookup:
• d["duck"] -> "eend"
• d["back"] # raises KeyError exception
• Delete, insert, overwrite:
• del d["water"] # {"duck": "eend", "back": "rug"}
• d["back"] = "rug" # {"duck": "eend", "back": "rug"}
• d["duck"] = "duik" # {"duck": "duik", "back": "rug"}
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More dictionary ops
• Keys, values, items:
• d.keys() -> ["duck", "back"]
• d.values() -> ["duik", "rug"]
• d.items() -> [("duck","duik"), ("back","rug")]
• Presence check:
• d.has_key("duck") -> 1; d.has_key("spam") -> 0
• Values of any type; keys almost any
• {"name":"Guido", "age":43, ("hello","world"):1,
42:"yes", "flag": ["red","white","blue"]}
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Dictionary details
• Keys must be immutable:
– numbers, strings, tuples of immutables
• these cannot be changed after creation
– reason is hashing (fast lookup technique)
– not lists or other dictionaries
• these types of objects can be changed "in place"
– no restrictions on values
• Keys will be listed in arbitrary order
– again, because of hashing
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Tuples
• key = (lastname, firstname)
• point = x, y, z
# paren’s optional
• x, y, z = point
• lastname = key[0]
• singleton = (1,)
# trailing comma!
• empty = ()
# parentheses!
• tuples vs. lists; tuples immutable
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Variables
• No need to declare
• Need to assign (initialize)
• use of uninitialized variable raises exception
• Not typed
if friendly: greeting = "hello world"
else: greeting = 12**2
print greeting
• Everything is a variable:
• functions, modules, classes
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Reference semantics
• Assignment manipulates references
• x = y does not make a copy of y
• x = y makes x reference the object y references
• Very useful; but beware!
• Example:
>>> a = [1, 2, 3]; b = a
>>> a.append(4); print b
[1, 2, 3, 4]
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Changing a shared list
a = [1, 2, 3]
a
1
2
3
1
2
3
1
2
3
a
b=a
b
a
a.append(4)
4
b
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Changing an integer
a=1
a
1
a
b=a
1
b
a
new int object created
by add operator (1+1)
2
a = a+1
b
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old reference deleted
by assignment (a=...)
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Control structures
if condition:
while condition:
statements
statements
[elif condition:
statements] ...
else:
for var in sequence:
statements
statements
break
continue
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Grouping indentation
In Python:
In C:
for i in range(20):
for (i = 0; i < 20; i++)
if i%3 == 0:
{
print i
if (i%3 == 0) {
if i%5 == 0:
printf("%d\n", i);
print "Bingo!"
if (i%5 == 0) {
print "---"
printf("Bingo!\n"); }
}
printf("---\n");
}
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Bingo!
------3
------6
------9
------12
------15
Bingo!
------18
-----
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Functions, procedures
def name(arg1, arg2, ...):
"documentation"
# optional
statements
return
# from procedure
return expression
# from function
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Example function
def gcd(a, b):
"greatest common divisor"
while a != 0:
a, b = b%a, a
# parallel assignment
return b
>>> gcd.__doc__
'greatest common divisor'
>>> gcd(12, 20)
4
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Classes
class name:
"documentation"
statements
-orclass name(baseclass1, baseclass2, ...):
...
Typically, statements contains method definitions:
def name(self, arg1, arg2, ...):
...
May also contain class variable assignments
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Example class
class Stack:
"A well-known data structure…"
def __init__(self):
# constructor
self.items = []
def push(self, x):
self.items.append(x)
# the sky is the limit
def pop(self):
x = self.items[-1]
# what happens if it’s empty?
del self.items[-1]
return x
def empty(self):
return len(self.items) == 0
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# Boolean result
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Using classes
• To create an instance, simply call the class object:
x = Stack()
# no 'new' operator!
• To use methods of the instance, call using dot notation:
x.empty()
# -> 1
x.push(1)
x.empty()
# [1]
# -> 0
x.push("hello")
x.pop()
# [1, "hello"]
# -> "hello"
# [1]
• To inspect instance variables, use dot notation:
x.items
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# -> [1]
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Subclassing
class FancyStack(Stack):
"stack with added ability to inspect inferior stack items"
def peek(self, n):
"peek(0) returns top; peek(-1) returns item below that; etc."
size = len(self.items)
assert 0 <= n < size
# test precondition
return self.items[size-1-n]
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Subclassing (2)
class LimitedStack(FancyStack):
"fancy stack with limit on stack size"
def __init__(self, limit):
self.limit = limit
FancyStack.__init__(self)
# base class constructor
def push(self, x):
assert len(self.items) < self.limit
FancyStack.push(self, x)
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# "super" method call
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Class & instance variables
class Connection:
verbose = 0
# class variable
def __init__(self, host):
self.host = host
# instance variable
def debug(self, v):
self.verbose = v
# make instance variable!
def connect(self):
if self.verbose:
# class or instance variable?
print "connecting to", self.host
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Instance variable rules
• On use via instance (self.x), search order:
– (1) instance, (2) class, (3) base classes
– this also works for method lookup
• On assigment via instance (self.x = ...):
– always makes an instance variable
• Class variables "default" for instance variables
• But...!
– mutable class variable: one copy shared by all
– mutable instance variable: each instance its own
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Modules
• Collection of stuff in foo.py file
– functions, classes, variables
• Importing modules:
– import string; print string.join(L)
– from string import join; print join(L)
• Rename after import:
– import string; s = string; del string
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Packages
• Collection of modules in directory
• Must have __init__.py file
• May contain subpackages
• Import syntax:
– from P.Q.M import foo; print foo()
– from P.Q import M; print M.foo()
– import P.Q.M; print P.Q.M.foo()
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Catching exceptions
def foo(x):
return 1.0/x
def bar(x):
try:
print foo(x)
except ZeroDivisionError, message:
print "Can’t divide by zero:", message
bar(0)
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Try-finally: cleanup
f = open(file)
try:
process_file(f)
finally:
f.close()
# always executed
print "OK" # executed on success only
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Raising exceptions
• raise IndexError
• raise IndexError("k out of range")
• raise IndexError, "k out of range"
• try:
something
except: # catch everything
print "Oops"
raise # reraise
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More on exceptions
• User-defined exceptions
– subclass Exception or any other standard exception
• Old Python: exceptions can be strings
– WATCH OUT: compared by object identity, not ==
• Last caught exception info:
– sys.exc_info() == (exc_type, exc_value, exc_traceback)
• Last uncaught exception (traceback printed):
– sys.last_type, sys.last_value, sys.last_traceback
• Printing exceptions: traceback module
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File objects
• f = open(filename[, mode[, buffersize])
– mode can be "r", "w", "a" (like C stdio); default "r"
– append "b" for text translation mode
– append "+" for read/write open
– buffersize: 0=unbuffered; 1=line-buffered; buffered
• methods:
– read([nbytes]), readline(), readlines()
– write(string), writelines(list)
– seek(pos[, how]), tell()
– fileno(), flush(), close()
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Standard library
• Core:
– os, sys, string, getopt, StringIO, struct, pickle, ...
• Regular expressions:
– re module; Perl-5 style patterns and matching rules
• Internet:
– socket, rfc822, httplib, htmllib, ftplib, smtplib, ...
• Miscellaneous:
– pdb (debugger), profile+pstats
– Tkinter (Tcl/Tk interface), audio, *dbm, ...
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Python CGI programming
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Outline
• HTML forms
• Basic CGI usage
• Setting up a debugging framework
• Security
• Handling persistent data
• Locking
• Sessions
• Cookies
• File upload
• Generating HTML
• Performance
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A typical HTML form
<form method="POST" action="http://host.com/cgi-bin/test.py">
<p>Your first name: <input type="text" name="firstname">
<p>Your last name: <input type="text" name="lastname">
<p>Click here to submit form: <input type="submit" value="Yeah!">
<input type="hidden" name="session" value="1f9a2">
</form>
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A typical CGI script
#!/usr/local/bin/python
import cgi
def main():
print "Content-type: text/html\n"
form = cgi.FieldStorage()
# parse query
if form.has_key("firstname") and form["firstname"].value != "":
print "<h1>Hello", form["firstname"].value, "</h1>"
else:
print "<h1>Error! Please enter first name.</h1>"
main()
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CGI script structure
• Check form fields
– use cgi.FieldStorage class to parse query
• takes care of decoding, handles GET and POST
• "foo=ab+cd%21ef&bar=spam" -->
{'foo': 'ab cd!ef', 'bar': 'spam'} # (well, actually, ...)
• Perform action
– this is up to you!
– database interfaces available
• Generate HTTP + HTML output
– print statements are simplest
– template solutions available
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Structure refinement
form = cgi.FieldStorage()
if not form:
...display blank form...
elif ...valid form...:
...perform action, display results (or next form)...
else:
...display error message (maybe repeating form)...
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FieldStorage details
• Behaves like a dictionary:
– .keys(), .has_key()
# but not others!
– dictionary-like object ("mapping")
• Items
– values are MiniFieldStorage instances
• .value gives field value!
– if multiple values: list of MiniFieldStorage instances
• if type(...) == types.ListType: ...
– may also be FieldStorage instances
• used for file upload (test .file attribute)
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Other CGI niceties
• cgi.escape(s)
– translate "<", "&", ">" to "&lt;", "&amp;", "&gt"
• cgi.parse_qs(string, keep_blank_values=0)
– parse query string to dictionary {"foo": ["bar"], ...}
• cgi.parse([file], ...)
– ditto, takes query string from default locations
• urllib.quote(s), urllib.unquote(s)
– convert between "~" and "%7e" (etc.)
• urllib.urlencode(dict)
– convert dictionary {"foo": "bar", ...} to query string
"foo=bar&..." # note asymmetry with parse_qs() above
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Dealing with bugs
• Things go wrong, you get a traceback...
• By default, tracebacks usually go to the
server's error_log file...
• Printing a traceback to stdout is tricky
– could happen before "Content-type" is printed
– could happen in the middle of HTML markup
– could contain markup itself
• What's needed is a...
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Debugging framework
import cgi
def main():
print "Content-type: text/html\n" # Do this first
try:
import worker
# module that does the real work
except:
print "<!-- --><hr><h1>Oops. An error occurred.</h1>"
cgi.print_exception() # Prints traceback, safely
main()
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Security notes
• Watch out when passing fields to the shell
– e.g. os.popen("finger %s" % form["user"].value)
– what if the value is "; cat /etc/passwd" ...
• Solutions:
– Quote:
• user = pipes.quote(form["user"].value)
– Refuse:
• if not re.match(r"^\w+$", user): ...error...
– Sanitize:
• user = re.sub(r"\W", "", form["user"].value)
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Using persistent data
• Store/update data:
– In plain files (simplest)
• FAQ wizard uses this
– In a (g)dbm file (better performance)
• string keys, string values
– In a "shelf" (stores objects)
• avoids parsing/unparsing the values
– In a real database (if you must)
• 3rd party database extensions available
• not my field of expertise
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Plain files
key = ...username, or session key, or whatever...
try:
f = open(key, "r")
data = f.read()
# read previous data
f.close()
except IOError:
data = ""
# no file yet: provide initial data
data = update(data, form)
# do whatever must be done
f = open(key, "w")
f.write(data)
# write new data
f.close()
# (could delete the file instead if updated data is empty)
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(G)DBM files
# better performance if there are many records
import gdbm
key = ...username, or session key, or whatever...
db = gdbm.open("DATABASE", "w")
# open for reading+writing
if db.has_key(key):
data = db[key]
# read previous data
else:
data = ""
# provide initial data
data = update(data, form)
db[key] = data
# write new data
db.close()
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Shelves
# a shelf is a (g)dbm files that stores pickled Python objects
import shelve
class UserData: ...
key = ...username, or session key, or whatever...
db = shelve.open("DATABASE", "w")
# open for reading+writing
if db.has_key(key):
data = db[key]
# an object!
else:
data = UserData(key)
# create a new instance
data.update(form)
db[key] = data
db.close()
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Locking
• (G)DBM files and shelves are not protected
against concurrent updates!
• Multiple readers, single writer usually OK
– simplest approach: only lock when writing
• Good filesystem-based locking is hard
– no cross-platform solutions
– unpleasant facts of life:
• processes sometimes die without unlocking
• processes sometimes take longer than expected
• NFS semantics
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A simple lock solution
import os, time
def unlock(self):
assert self.locked
class Lock:
self.locked = 0
os.rmdir(self.filename)
def __init__(self, filename):
self.filename = filename
# auto-unlock when lock object is deleted
self.locked = 0
def __del__(self):
if self.locked:
def lock(self):
self.unlock()
assert not self.locked
while 1:
try:
# for a big production with timeouts,
os.mkdir(self.filename)
# see the Mailman source code (LockFile.py);
self.locked = 1
# it works on all Unixes and supports NFS;
return
# but not on Windows,
# or break
except os.error, err:
# and the code is very complex...
time.sleep(1)
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Sessions
• How to correlate requests from same user?
– Assign session key on first contact
– Incorporate session key in form or in URL
– In form: use hidden input field:
• <input type="hidden" name="session" value="1f9a2">
– In URL:
• http://myhost.com/cgi-bin/myprog.py/1f9a2
• passed in environment (os.environ[...]):
– PATH_INFO=/1f9a2
– PATH_TRANSLATED=<rootdir>/1f9a2
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Cookies
• How to correlate sessions from the same user?
– Store "cookie" in browser
• controversial, but useful
– Module: Cookie.py (Tim O'Malley)
• writes "Set-Cookie" headers
• parses HTTP_COOKIE environment variable
– Note: using cookies affects our debug framework
• cookies must be printed as part of HTTP headers
• cheapest solution:
– move printing of blank line into worker module
– (and into exception handler of debug framework)
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Cookie example
import os, cgi, Cookie
c["user"] = user
c = Cookie.Cookie()
print c
try:
c.load(os.environ["HTTP_COOKIE"])
except KeyError:
pass
form = cgi.FieldStorage()
try:
user = form["user"].value
except KeyError:
try:
user = c["user"].value
except KeyError:
user = "nobody"
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print """
<form action="/cgi-bin/test.py"
method="get">
<input type="text" name="user"
value="%s">
</form>
""" % cgi.escape(user)
# debug: show the cookie header we wrote
print "<pre>"
print cgi.escape(str(c))
print "</pre>"
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File upload example
import cgi
form = cgi.FieldStorage()
if not form:
print """
<form action="/cgi-bin/test.py" method="POST" enctype="multipart/form-data">
<input type="file" name="filename">
<input type="submit">
</form>
"""
elif form.has_key("filename"):
item = form["filename"]
if item.file:
data = item.file.read()
# read contents of file
print cgi.escape(data)
# rather dumb action
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Generating HTML
• HTMLgen (Robin Friedrich)
http://starship.python.net/crew/friedrich/HTMLgen/html/main.html
>>> print H(1, "Chapter One")
<H1>Chapter One</H1>
>>> print A("http://www.python.org/", "Home page")
<A HREF="http://www.python.org/">Home page</A>
>>> # etc. (tables, forms, the works)
• HTMLcreate (Laurence Tratt)
http://www.spods.dcs.kcl.ac.uk/~laurie/comp/python/htmlcreate/
• not accessible at this time
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CGI performance
• What causes slow response?
– One process per CGI invocation
• process creation (fork+exec)
• Python interpreter startup time
• importing library modules (somewhat fixable)
– Connecting to a database!
• this can be the killer if you use a real database
– Your code?
• probably not the bottleneck!
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Avoiding fork()
• Python in Apache (mod_pyapache)
• problems: stability; internal design
• advantage: CGI compatible
• may work if CGI scripts are simple and trusted
• doesn't avoid database connection delay
• Use Python as webserver
• slow for static content (use different port)
• advantage: total control; session state is easy
• FastCGI, HTTPDAPI etc.
• ZOPE
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ZOPE
• Z Object Publishing Environment
– http://www.zope.org
– complete dynamic website management tool
• written in cross-platform Python; Open Source
– http://host/path/to/object?size=5&type=spam
• calls path.to.object(size=5, type="spam")
– DTML: templatized HTML (embedded Python code)
– ZOBD (Z Object DataBase; stores Python objects)
• transactionsm selective undo, etc.
– etc., etc.
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Case study
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FAQ wizard
• Tools/faqwiz/faqwiz.py in
Python distribution
• http://www.python.org
/cgi-bin/faqw.py
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faqw.py - bootstrap
import os, sys
try:
FAQDIR = "/usr/people/guido/python/FAQ"
SRCDIR = "/usr/people/guido/python/src/Tools/faqwiz"
os.chdir(FAQDIR)
sys.path.insert(0, SRCDIR)
import faqwiz
except SystemExit, n:
sys.exit(n)
except:
t, v, tb = sys.exc_type, sys.exc_value, sys.exc_traceback
print
import cgi
cgi.print_exception(t, v, tb)
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faqwiz.py - main code
class FaqWizard:
def go(self):
print 'Content-type: text/html'
def __init__(self):
req = self.ui.req or 'home'
self.ui = UserInput()
mname = 'do_%s' % req
self.dir = FaqDir()
try:
meth = getattr(self, mname)
def do_home(self):
self.prologue(T_HOME)
emit(HOME)
except AttributeError:
self.error("Bad request type %s." % `req`)
else:
try:
def do_search(self): ...
def do_index(self): ...
def do_roulette(self): ...
def do_show(self): ...
def do_edit(self): ...
def do_review(self): ...
def do_help(self): ...
meth()
except InvalidFile, exc:
self.error("Invalid entry file name %s" % exc.file)
except NoSuchFile, exc:
self.error("No entry with file name %s" % exc.file)
except NoSuchSection, exc:
self.error("No section number %s" % exc.section)
...etc...
self.epilogue()
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Example: do_roulette()
def do_roulette(self):
import random
files = self.dir.list()
if not files:
self.error("No entries.")
return
file = random.choice(files)
self.prologue(T_ROULETTE)
emit(ROULETTE)
self.dir.show(file)
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Persistency
• All data stored in files (faqNN.MMM.htp)
• Backed up by RCS files (RCS/faqNN.MMM.htp,v)
– RCS logs and diffs viewable
• RCS commands invoked with os.system() or os.popen()
• search implemented by opening and reading each file
• NO LOCKING!
– infrequent updates expected
• in practice, one person makes most updates :-)
– one historic case of two users adding an entry to the same
section at the same time; one got an error back
– not generally recommended
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faqconf.py, faqcust.py
• faqconf.py defines named string constants for
every bit of output generated by faqwiz.py
– designed for customization (e.g. i18n)
– so you can customize your own faq wizard
– e.g. OWNEREMAIL = "[email protected]"
– this includes the list of sections in your faq :-(
• faqcust.py defines overrides for faqconf.py
– so you don't need to edit faqwiz.py
• to make it easier to upgrade to newer faqwiz version
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Webchecker
• Tools/webchecker/webchecker.py in Python distribution
• Not a CGI application but a web client application
– while still pages to do:
• request page via http
• parse html, collecting links
– pages once requested won't be requested again
– links outside original tree treated as leaves
• existence checked but links not followed
– reports on bad links
• what the bad URL is
• on which page(s) it is referenced
– could extend for other reporting
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Reference URLs
• Python websites
– http://www.python.org (official site)
– http://starship.python.net (community)
• Python web programming topic guide
– http://www.python.org/topics/web/
• These slides on the web (soon)
– http://www.python.org/doc/essays/ppt/sd99east.ppt
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Reference books
• http://www.python.org/psa/bookstore/
• 1996
– Programming Python (Lutz)
– [Internet Programming with Python (Watters e.a.)]
• 1998
– Python Pocket Reference (Lutz)
• 1999
– Learning Python (Lutz, Ascher)
– Python: Essential Reference (Beazley)
– Quick Python Book (Harms, McDonald)
• Expected 1999/2000
– Win 32, Tkinter, teach-yourself-in-24-hrs, annotated archives, ...
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Any questions?
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Nobody expects the
Spanish Inquisition!
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