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A Lightning Tour of Python
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Mike Coleman
Math Dogs, LLC
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mkc@mathdogs.com
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Python Is...
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interpreted, interactive OO scripting language
comes from Unix background, runs on Win, Mac, etc
elegant notation
readable, maintainable code
small language--details pushed into libraries
re Perl, less emphasis on text processing, more on large-scale, general-purpose programming
introspective
garbage collection by ref counting (cycle-breaking in v2.0)
threading (native under Linux)
my biases (favorite languages, minimalism)
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Python Is Not...
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the choice of PHBs
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Pardon My Whining
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# print a hash of hashes
foreach $family { keys %HoH } {
print "$family: ";
foreach $role { keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "\n";
}
# pop arrays in list, return list of popped elements
@tailings = popmany ( \@a, \@b, \@c, \@d );
sub popmany {
my $aref;
my @retlist = ();
foreach $aref ( @_ ) {
push @retlist, pop @$aref;
}
return @retlist;
}
What
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is
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this stuff?!?
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Python Philosophy
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
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There should be one--and preferably only one--obvious way to do it.
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-- excerpted from Tim Peters' Zen of Python
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A Simple Program
In file 'hello.py':
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#!/usr/bin/python
import sys
# a comment
for n in sys.argv:
print 'Hello %s!' % n
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which does this
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$ ./hello.py Guido Larry
Hello Guido!
Hello Larry!
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(note block indentation in for loop body)
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Indentation
C style:
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if (x > 0) {
if (y > 0) {
blah();
blah();
}
}
else
blah();
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Indentation
Python style:
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if x > 0 :
if y > 0 :
blah()
blah()
else:
blah()
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Interactive Use
The Python interpreter can be used interactively, typically to try things out:
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$
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python
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Python 2.0 (#0, Feb 21 2001, 12:48:36)
[GCC 2.95.3 20010125 (prerelease)] on linux2
Type "copyright", "credits" or "license" for more information.
>>>
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x = 1
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>>>
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print x
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1
>>>
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a, b = 0, 1
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>>>
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while b < 1000:
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...
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print b,
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...
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a, b = b, a+b
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...
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
>>>
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Basic Types
Numbers
Int (>= 32 bits)
Long (arbitrary precision)
Float (double-precision native)
Complex (pair of floats)
None
Sequences
String (plus Unicode strings in 2.0)
List (mutable)
Tuple (non-mutable)
Mapping (hash on non-mutable keys)
Callable types (e.g., functions, lambdas, class methods)
File
Module
Class
Instance
Internal types (stack frame, compiled code, etc)
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Literals
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int = 1
long = 1234123412341234123412341234L
float = 0.9e10
complex = 1.8 + 3.2J
string = "he said, \"hooray\""
string = 'line one\nline two'
string = """line one
line two
line three
"""
string = r'[a-zA-Z][\w-]*'
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Literals (cont.)
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list = []
list = [1, 2, 3]
list = [1, 2, ['foo', bar']]
tuple = ()
tuple = (1,)
tuple = (1, 2, 'baz', [5, 6])
dict = { 'one' : 1, 'two' : 2 }
func = lambda x, y: x + y
x = func(1, 2) # x gets 3
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Control Structures - Loops
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while 1: # loop forever
pass # do nothing
for c in "letters":
handle_one_letter(c)
for n in [0, 1, 2, 3]:
handle_one_number(n)
for n in range(4): # or xrange
handle_one_number(n)
for name, age in [("Homer", 37), ("Marge, 36)]:
print name, age
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Control Structures - Conditionals
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if x > 0:
foo(x)
elif x < 0:
bar(x)
else:
baz(x)
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no switch statement
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assert x > 0, "uh-oh, x (= %s) not > 0!" % x
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assertion checking disabled by -O flag
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Control Structures - Exceptions
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try:
f = open('foo')
except IOError, e:
print 'Unable to open 'foo': ', e
except TypeError, e:
print "Shouldn't happen: ", e
else:
# do this if there were no exceptions
data = f.read()
f.close()
f = open('foo')
try:
# do something
finally:
# do this even if above causes exception
f.close()
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finally and except cannot appear together
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Functions
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def draw_square(width, center=(0,0)):
"draw a square on the display"
for side in range(4):
draw_side(side, width, center)
# ...
return (time, cost)
#invoked like this:
draw_square(4, (5, 6))
mytime, mycost = draw_square(7)
draw_square(center=(4,2), width=7)
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Functions (cont.)
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def printf(format, *args):
# args is a tuple of all args except the first
for a in args:
# handle arg a
def foo(**keyargs):
# keyargs is a dictionary of keyword->value pairs
for k in keyargs.keys():
# handle keyword arg with
# key k and arg keyargs[k]
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either or both of * and ** can appear in a parameter list
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Operators
The usual:
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+ - * / % ~ >> << & | ^ < > <= >= == !=
[] # indexing
o.a # object attribute
** # power
not and or # short circuit, 2 or 3 -> 2
x is y, x is not y # object identity
x in y, x not in y # contains
"abc" + "def" -> "abcdef"
"abc" * 3 -> "abcabcabc"
[1,2] * 3 -> [1,2,1,2,1,2]
"%d %s %f" % (1, 'foo', 3.5) -> "1 foo 3.500000"
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version 2.0 has augmented assignment ops like +=
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Operators - Slicing
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a = [1,2,3,4,5]
a[1] = 6 # a == [1,6,3,4,5]
a[2:4] = [8,9] # a == [1,6,8,9,5]
a[3:5] = [2,3,4] # a == [1,6,8,2,3,4]
a[2:] = [0] # a == [1,6,0]
a[:-1] = [2,3,4] # a == [2,3,4,0]
del a[1] # a == [2,4,0]
del a[-2:] # a == [2]
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References
in Python, everything is a reference; this mainly makes a difference only for mutable types (lists, dictionaries):
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a = [1,2,3,4,5]
b = a
a[0] = 6 # b[0] == 6
c = a[:] # shallow copy
a[0] = 7 # c[0] == 6
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library functions for shallow/deep copying
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Applicative Functions
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a = [1,2,3,4,5]
b = [0,1,0,0,1]
def p(x,y):
return x*y
c = map(p, a, b)
# c == [0,2,0,0,5]
d = reduce(p, a)
# d == 120
e = filter(lambda x: x > 3, a)
# e == [4,5]
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List Comprehensions (new in 2.0)
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a = [1,2,3,4]
b = ['a', 'b', 'c']
c = [ (m, 2*n) for n in a for m in b if n > 1 ]
# c == [('a', 4), ('b', 4), ('c', 4),
# ('a', 6), ('b', 6), ('c', 6),
# ('a', 8), ('b', 8), ('c', 8)]
# Given the list L, make a list of all strings
# containing the substring S.
S, L = 'ed', ['walked', 'foo', 'barred']
sublist = filter(lambda s, substring=S:
string.find(s, substring) != -1,
L)
sublist = [ s for s in L if string.find(s, S) != -1 ]
# L == ['walked', 'barred']
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File I/O
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f = open("foo", "w") # open for writing
s = f.read(n) # read n bytes
s = f.readline() # read a line
l = f.readlines() # snarf whole file
f.write(s)
f.writelines(l) # write list of
# strings
# plus the other usual functions
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standard files are sys.stdin, sys.stdout, sys.stderr
pickle and shelve serialize to file and dbm
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Modules
means for breaking up code into separate files
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# file foo.py
x = 1
def bar(y):
x = x + y
# in other code
import foo
foo.bar(3)
c = foo.x # gets 4
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PYTHONPATH controls module search path
also could be foo.so, a C extension module
packages allow grouping of modules into directories
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Classes
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class Account:
"a simple account class"
acct_type = 'Basic' # class variable
def __init__(self, name): # method
self.name = name # instance variable
self.__balance = 0 # likewise, hidden
def deposit(self, amount):
self.__balance = self.__balance + amount
a = Account('a name')
a.deposit(20)
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no class methods
multiple inheritance
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Classes - Operator Overloading
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class MyNumber:
"my number class"
def __init__(self, value):
self.v = int(value)
def __add__(self, value):
return MyNumber(value + self.v)
n1 = MyNumber(3)
n2 = MyNumber(4)
n3 = n1 + n2
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__op__ methods for all ops
other features for full support
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Libraries
maybe 150 (?) modules in the standard Python Library.
for example:
math
re
string
struct
anydbm (multiple dbm-ish modules)
fileinput (like Perl's "while(<>)")
os
signal
rexec
xml (standard in v2.0)
more, less-blessed stuff in the Vaults of Parnassus (like CPAN)
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Large-Scale Software Development
classes
cooperative hiding
modules
external modules
packages
assertions
PyUnit
doctest (in v2.0)
distutils
%system "killall xclock"
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SUBTERFUGUE
tool to monitor and mess with the reality of programs
"strace" meets "expect"
implemented in Python
several thousand lines of Python
parts in C as extension module, for speed
class interface for "tricks" (like plugins)
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SUBTERFUGUE
Here is a regular xclock.
"xclock -update 1"
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%xsystem "xclock -geometry %30x30+25+60 -update 1 -bg black -fg blue -hands green"
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SUBTERFUGUE
Here is an xclock run under the TimeWarp trick.
"sf --trick=TimeWarp:w=50.0 xclock -update 1"
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%xsystem "sf --tri=TimeWarp:w=50.0 xclock -geometry %30x30+25+60 -update 1 -bg black -fg blue -hands green"
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SUBTERFUGUE - TimeWarp.py (1)
%system "killall xclock"
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class TimeWarp(Trick):
def __init__(self, options):
self.options = options
self.w = 1.0
self.delta = 0
self.start_t = time.time()
if options.has_key('w'):
self.w = float(options['w'])
if self.w <= 0:
sys.exit("error: %s: 'w' must be"
+ " positive\nusage:%s"
% (self.__class__.__name__,
self.usage()))
if options.has_key('now'):
self.delta = float(options['now']) - self.start_t
if options.has_key('delta'):
self.delta = self.delta + float(options['delta'])
def callmask(self):
return { 'time' : 1, 'gettimeofday' : 1, 'ftime' : 1,
'nanosleep' : 1, 'select' : 1, 'poll' : 1, }
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SUBTERFUGUE - TimeWarp.py (2)
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def callbefore(self, pid, call, args):
if call == 'time':
return (args[0], None, None, None)
elif call == 'gettimeofday':
return (args[0], None, None, None)
elif call == 'select':
timeout = args[4]
if timeout:
if _warp_timeval(pid, self.w, timeout):
return (timeout, None, None, None)
elif call == 'poll':
if args[2] > 0:
return (None, None, None, (args[0], args[1],
args[2] / self.w))
elif call == 'nanosleep':
oldtimeval = _warp_timeval(pid, self.w, args[0], 1000)
if oldtimeval:
return ((oldtimeval, args), None, None, None)
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SUBTERFUGUE - TimeWarp.py (3)
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def callafter(self, pid, call, result, state):
now = time.time()
delta_t = now - self.start_t
now_w = self.start_t + delta_t * self.w + self.delta
if call == 'time':
now_wi = int(now_w)
if state:
ptrace.pokedata(pid, state, now_wi)
return now_wi
elif call == 'gettimeofday' and result != -1:
_put_timeval(pid, state, now_w)
elif call == 'select' and result != -1 and state:
_warp_timeval(pid, 1.0 / self.w, state)
elif call == 'nanosleep':
oldtimeval, args = state
if args[1]:
_warp_timeval(pid, 1.0 / self.w, args[1], 1000)
# save and restore it so it doesn't get stepped on
remaining = _get_timeval_2(pid, args[1])
_put_timeval_2(pid, args[0], oldtimeval) # here
if args[1]:
_put_timeval_2(pid, args[1], remaining)
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Web Programming - cgi
basic cgi functionality
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print "Content-Type: text/html"
print # blank line, end of headers
form = cgi.FieldStorage()
form_ok = 0
if form.has_key("name") and form.has_key("addr"):
form_ok = 1
if not form_ok:
print "Error
"
print "Please fill in the name and addr fields."
return
print "name:", form["name"].value
print "
addr:", form["addr"].value
# further form processing here
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Web Programming - urllib
generic url interface
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import urllib
params = urllib.urlencode({'spam': 1, 'eggs': 2,
'bacon': 0})
# GET
f = urllib.urlopen("http://www.musi-cal.com/"
+ "cgi-bin/query?%s"
% params)
# or POST
f = urllib.urlopen("http://www.musi-cal.com/"
+ "cgi-bin/query",
params)
print f.read()
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%PCACHE 0
Internet Protocols
protocol-specific modules ftplib, httplib, imaplib, poplib, nntplib, smtplib, telnetlib gopherlib
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>>> from ftplib import FTP
>>> ftp = FTP('ftp.cwi.nl')
>>> ftp.login() # user anonymous, passwd user@hostname
>>> ftp.retrlines('LIST') # list directory contents
total 24418
drwxrwsr-x 5 ftp-usr pdmaint 1536 Mar 20 09:48 .
dr-xr-srwt 105 ftp-usr pdmaint 1536 Mar 21 14:32 ..
-rw-r--r-- 1 ftp-usr pdmaint 5305 Mar 20 09:48 INDEX
>>> ftp.retrbinary('RETR README',
open('README', 'wb').write)
'226 Transfer complete.'
>>> ftp.quit()
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Web Programming - Higher Level Solutions
mod_python
basic, fast cgi interface
WebWare
application server
persistent process sits behind web server
like Java app servers
alpha/beta
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Web Programming - ZOPE
ZOPE
application server
ORB, persistent store
steep learning curve
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A Biased Summary
Python
simple
clear
minimal
"designed not accreted"
the way forward?
Perl
more mature
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(but Python is catching up)
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larger developer base
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(but Python is catching up)
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arcane
could go the way of tcl?
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A Parting Shot
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YODA:
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Code! Yes. A programmer's strength flows from code maintainability. But beware of Perl. Terse syntax... more than one way to do it... default variables. The dark side of code maintainability are they. Easily they flow, quick to join you when code you write. If once you start down the dark path, forever will it dominate your destiny, consume you it will.
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LUKE:
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Is Perl better than Python?
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YODA:
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No... no... no. Quicker, easier, more seductive.
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LUKE:
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But how will I know why Python is better than Perl?
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YODA:
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You will know. When your code you try to read six months from now.
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-- funkster@midwinter.com