11/12/1999© 1999 cnri, guido van rossum1 using python for cgi programming guido van rossum cnri...
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11/12/1999 © 1999 CNRI, Guido van Rossum 1
Using Python for CGI programming
Guido van RossumCNRI
(Corporation for National Research Initiatives, Reston, Virginia, USA)
11/12/1999 © 1999 CNRI, Guido van Rossum 2
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" # concatenation
• "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 # 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] # -> [98, "bottles", "of", "beer"]
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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 typedif 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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a
1 2 3
b
a
1 2 3
b
4
a = [1, 2, 3]
a.append(4)
b = a
a 1 2 3
Changing a shared list
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a
1
b
a
1b
a = 1
a = a+1
b = a
a 1
2
Changing an integer
old reference deletedby assignment (a=...)
new int object createdby add operator (1+1)
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Control structures
if condition:
statements
[elif condition:
statements] ...
else:
statements
while condition:
statements
for var in sequence:
statements
break
continue
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Grouping indentation
In Python:
for i in range(20):
if i%3 == 0:
print i
if i%5 == 0:
print "Bingo!"
print "---"
In C:
for (i = 0; i < 20; i++)
{
if (i%3 == 0) {
printf("%d\n", i);
if (i%5 == 0) {
printf("Bingo!\n"); }
}
printf("---\n");
}
0Bingo!---------3---------6---------9---------12---------15Bingo!---------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
-or-
class 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 # 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) # [1]
x.empty() # -> 0
x.push("hello") # [1, "hello"]
x.pop() # -> "hello" # [1]
• To inspect instance variables, use dot notation:x.items # -> [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) # "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: somethingexcept: # 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 "<", "&", ">"
• 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 solutionimport os, time
class Lock:
def __init__(self, filename):
self.filename = filename
self.locked = 0
def lock(self):
assert not self.locked
while 1:
try:
os.mkdir(self.filename)
self.locked = 1
return # or break
except os.error, err:
time.sleep(1)
def unlock(self):
assert self.locked
self.locked = 0
os.rmdir(self.filename)
# auto-unlock when lock object is deleted
def __del__(self):
if self.locked:
self.unlock()
# for a big production with timeouts,
# see the Mailman source code (LockFile.py);
# it works on all Unixes and supports NFS;
# but not on Windows,
# and the code is very complex...
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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 exampleimport os, cgi, Cookie
c = Cookie.Cookie()
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"
c["user"] = user
print c
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
import cgi
cgi.print_exception(t, v, tb)
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faqwiz.py - main codeclass FaqWizard:
def __init__(self):
self.ui = UserInput()
self.dir = FaqDir()
def do_home(self):
self.prologue(T_HOME)
emit(HOME)
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): ...
...etc...
def go(self):
print 'Content-type: text/html'
req = self.ui.req or 'home'
mname = 'do_%s' % req
try:
meth = getattr(self, mname)
except AttributeError:
self.error("Bad request type %s." % `req`)
else:
try:
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)
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 theSpanish Inquisition!