An Introduction to the Zen of Python Doug Hellmann February, 2011 Friday, February 18, 2011

The Zen of Python by Tim Peters •Beautiful is better than ugly. •Explicit is better than implicit. •Simple is better than complex. •Complex is better than complicated. •Flat is better than nested. •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. •In the face of ambiguity, refuse the temptation to guess. •There should be one – and preferably only one – obvious way to do it. •Although that way may not be obvious at first unless you're Dutch. •Now is better than never. •Although never is often better than *right* now. •If the implementation is hard to explain, it's a bad idea. •If the implementation is easy to explain, it may be a good idea. •Namespaces are one honking great idea – let's do more of those! Friday, February 18, 2011

Beautiful is better than ugly. • Write programs for human readers • Simple expression syntax • Consistent syntax and behavior Friday, February 18, 2011

1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Simple Expressions Friday, February 18, 2011

1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Simple Expressions Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Simple Expressions 1 i = 5 + 5 2 3 s = 'literal string' 4 5 m = """multi-line 6 string 7 """ 8 9 us = u'üñîçø∂é string' 10 11 vals = [ 1, 3, 5, 7, ] 12 vals[0] 13 14 t = ('a', 1, vals, None) 15 t[1] 16 17 d = { 'a':'a value', 18 'b':'b value', 19 } 20 d['a'] 21 22 if a or b: 23 do_something() 24 Friday, February 18, 2011

Explicit is better than implicit. • Boolean type • No “hidden” loop variables • Use “self” to refer to object inside a method Friday, February 18, 2011

Boolean Values 1 t = True 2 f = False 3 4 if t: 5 print 't is true' 6 7 if not f: 8 print 'f is false' 9 10 a = t or f 11 b = t and f 12 13 print a 14 print b $ python booleans.py t is true f is false True False Friday, February 18, 2011

for loops 1 print 'Numbers:' 2 3 for i in range(5): 4 print i 5 6 print 7 print 'Letters:' 8 9 for c in [ 'a', 'b', 'c' ]: 10 print c $ python for_loop.py Numbers: 0 1 2 3 4 Letters: a b c Friday, February 18, 2011

Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Simple is better than complex. • Garbage collection • print statement • Interactive interpreter for experimentation Friday, February 18, 2011

Printing 1 # -*- encoding: utf-8 -*- 2 3 print 'string:', 'string' 4 5 print 'unicode:', u'üñîçø∂é string' 6 7 print 'number:', 5 * 5 8 9 my_list = [ 1, 'a', None ] 10 print 'list:', my_list 11 12 my_dict = { 'a':'a value', 'b':'b value' } 13 print 'dict:', my_dict $ python printing.py string: string unicode: üñîçø∂é string number: 25 list: [1, 'a', None] dict: {'a': 'a value', 'b': 'b value'} Friday, February 18, 2011

Printing, with printf-style Formatting 1 # -*- encoding: utf-8 -*- 2 3 my_list = [ 1, 'a', None ] 4 my_dict = { 'a':'a value', 'b':'b value' } 5 6 args = ('string', u'üñîçø∂é string', 5 * 5, 7 my_list, my_dict) 8 9 print ''' 10 string : %s 11 unicode: %s 12 number : %d 13 list : %s 14 dict : %s 15 ''' % args $ python printing_with_format.py string : string unicode: üñîçø∂é string number : 25 list : [1, 'a', None] dict : {'a': 'a value', 'b': 'b value'} Friday, February 18, 2011

Interactive Interpreter Friday, February 18, 2011

Complex is better than complicated. • Do complex things in a “Pythonic” way • SciPy and NumPy for advanced science and math Friday, February 18, 2011

Interactive Graphic Environment SciPy, NumPy, matplotlib Friday, February 18, 2011

NumPy Performance 1 a = range(10000000) 2 b = range(10000000) 3 c = [] 4 for i in range(len(a)): 5 c.append(a[i] + b[i]) Standard Python 1 import numpy as np 2 a = np.arange(10000000) 3 b = np.arange(10000000) 4 c = a + b NumPy 5-10 seconds “nearly instantaneous” Friday, February 18, 2011

Flat is better than nested. • Wide variety of modules in standard library • Namespace is flat • No need to use long names: com.company.java.blah.blah Friday, February 18, 2011

Standard Library Modules: Networking • socket • select • SocketServer • BaseHTTPServer • asyncore • asynchat • xmlrpclib • SimpleXMLRPCServer Friday, February 18, 2011

Standard Library Modules: Internet Protocols • urllib • httplib • ftplib • smtpd • smtplib • poplib • imaplib • json Friday, February 18, 2011

Standard Library Modules: Command Line • getopt • optparse • argparse • fileinput • cmd • readline Friday, February 18, 2011

Standard Library Modules: Parallel Processing • subprocess • threading • multiprocessing • Queue Friday, February 18, 2011

Standard Library Modules: Databases • anydbm • pickle • shelve • sqlite3 Friday, February 18, 2011

Standard Library Modules: Miscellaneous • datetime • grp • pwd • profile • timeit • struct • ctypes • xml • tarfile • csv Friday, February 18, 2011

Sparse is better than dense. • Standard library stays “shallow” • http://pypi.python.org/ • Allows separate release schedules • pip (“pip installs Python packages”) Friday, February 18, 2011

Readability counts. • Whitespace for block structures • Minimal punctuation • Built-in documentation • Extensive online documentation at http://docs.python.org/ Friday, February 18, 2011

1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Code Blocks Friday, February 18, 2011

1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Code Blocks Friday, February 18, 2011

Minimal Punctuation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Minimal Punctuation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Inline Documentation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Inline Documentation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something() Friday, February 18, 2011

Special cases aren’t special enough to break the rules. • Everything is an object • Methods and functions differ by scope • Dynamic typing and runtime attribute lookup • Most “features” in external modules Friday, February 18, 2011

String Methods 1 print ' string value '.split() 2 print ' string value '.strip() 3 print 'str' in ' string value ' 4 print 'CHUGALUG'.startswith('C') 5 print 'CHUGALUG'.endswith('end') 6 print ':'.join(['a', 'b', 'c']) $ python string_methods.py ['string', 'value'] string value True True False a:b:c Friday, February 18, 2011

String Methods 1 print ' string value '.split() 2 print ' string value '.strip() 3 print 'str' in ' string value ' 4 print 'CHUGALUG'.startswith('C') 5 print 'CHUGALUG'.endswith('end') 6 print ':'.join(['a', 'b', 'c']) $ python string_methods.py ['string', 'value'] string value True True False a:b:c Friday, February 18, 2011

String Methods 1 print ' string value '.split() 2 print ' string value '.strip() 3 print 'str' in ' string value ' 4 print 'CHUGALUG'.startswith('C') 5 print 'CHUGALUG'.endswith('end') 6 print ':'.join(['a', 'b', 'c']) $ python string_methods.py ['string', 'value'] string value True True False a:b:c Friday, February 18, 2011

String Methods 1 print ' string value '.split() 2 print ' string value '.strip() 3 print 'str' in ' string value ' 4 print 'CHUGALUG'.startswith('C') 5 print 'CHUGALUG'.endswith('end') 6 print ':'.join(['a', 'b', 'c']) $ python string_methods.py ['string', 'value'] string value True True False a:b:c Friday, February 18, 2011

String Methods 1 print ' string value '.split() 2 print ' string value '.strip() 3 print 'str' in ' string value ' 4 print 'CHUGALUG'.startswith('C') 5 print 'CHUGALUG'.endswith('end') 6 print ':'.join(['a', 'b', 'c']) $ python string_methods.py ['string', 'value'] string value True True False a:b:c Friday, February 18, 2011

String Methods 1 print ' string value '.split() 2 print ' string value '.strip() 3 print 'str' in ' string value ' 4 print 'CHUGALUG'.startswith('C') 5 print 'CHUGALUG'.endswith('end') 6 print ':'.join(['a', 'b', 'c']) $ python string_methods.py ['string', 'value'] string value True True False a:b:c Friday, February 18, 2011

String Methods 1 print ' string value '.split() 2 print ' string value '.strip() 3 print 'str' in ' string value ' 4 print 'CHUGALUG'.startswith('C') 5 print 'CHUGALUG'.endswith('end') 6 print ':'.join(['a', 'b', 'c']) $ python string_methods.py ['string', 'value'] string value True True False a:b:c Friday, February 18, 2011

Function and Method Scope 1 class MyClass(object): 2 3 def do_something(self, arg1, arg2): 4 return arg1 * arg2 5 6 def my_function(arg1, arg2): 7 return arg1 * arg2 Friday, February 18, 2011

Dynamic Attribute Lookup 1 class MyClass(object): 2 3 CLASS_ATTR = 'class value' 4 5 def __init__(self, arg): 6 self.attr = arg 7 8 o = MyClass('instance value') 9 print o.attr 10 print o.CLASS_ATTR $ python dynamic_attribute_lookup.py instance value class value Friday, February 18, 2011

Dynamic Typing 1 class A(object): 2 def do_something(self): 3 print 'in A' 4 5 class B(object): 6 def do_something(self): 7 print 'in B' 8 9 def work_on_obj(c): 10 c.do_something() 11 12 a = A() 13 b = B() 14 15 work_on_obj(a) 16 work_on_obj(b) $ python dynamic_typing.py in A in B Friday, February 18, 2011

Regular Expressions 1 import re 2 3 pattern = 'this' 4 text = 'Does this text match the pattern?' 5 6 match = re.search(pattern, text) 7 8 s = match.start() 9 e = match.end() 10 11 print 'Found :', match.re.pattern 12 print 'In :', match.string 13 print 'Range :', s, '-', e 14 print 'Substring:', text[s:e] $ python using_regex.py Found : this In : Does this text match the pattern? Range : 5 - 9 Substring: this Friday, February 18, 2011

Although practicality beats purity. • Multiple Programming Models • OOP • Procedural • Functional Friday, February 18, 2011

Procedural 1 def my_function(arg1, arg2): 2 """This function multiplies arg1 by arg2. 3 """ 4 return arg1 * arg2 5 6 results = [] 7 for i in range(5): 8 results.append(my_function(i, i)) 9 10 print results $ python procedural.py [0, 1, 4, 9, 16] Friday, February 18, 2011

Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16] Friday, February 18, 2011

Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16] Friday, February 18, 2011

Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16] Friday, February 18, 2011

Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16] Friday, February 18, 2011

Functional: Generator Expression 1 results = ( i*i for i in range(5) ) 2 3 print results 4 5 for i in results: 6 print i 1 $ python generator_expr.py 2 at 0x1004d20a0> 3 0 4 1 5 4 6 9 7 16 Friday, February 18, 2011

Functional: Generators 1 def gen_values(n): 2 for i in range(n): 3 yield i*i 4 5 print gen_values(5) 6 7 for i in gen_values(5): 8 print i 1 $ python generator.py 2 3 0 4 1 5 4 6 9 7 16 Friday, February 18, 2011

Errors should never pass silently. • Exception-based error handling • Tracebacks aid debugging Friday, February 18, 2011

Exceptions and Tracebacks 1 def f(n): 2 if n > 0: 3 return n * f(n-1) 4 return 1 5 6 print f('5') $ python traceback.py Traceback (most recent call last): File ".../traceback.py", line 6, in print f('5') File ".../traceback.py", line 3, in f return n * f(n-1) TypeError: unsupported operand type(s) for -: 'str' and 'int' Friday, February 18, 2011

Raising Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 print f('5') $ python raising.py Traceback (most recent call last): File ".../raising.py", line 8, in print f('5') File ".../raising.py", line 3, in f raise TypeError('n should be an int') TypeError: n should be an int Friday, February 18, 2011

Raising Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 print f('5') $ python raising.py Traceback (most recent call last): File ".../raising.py", line 8, in print f('5') File ".../raising.py", line 3, in f raise TypeError('n should be an int') TypeError: n should be an int Friday, February 18, 2011

Unless explicitly silenced. • Catch exceptions with try:except • Process • Convert • Ignore Friday, February 18, 2011

Catching Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 try: 9 print f('5') 10 except TypeError, err: 11 print 'Had an error:', err 12 else: 13 print 'No error' 14 finally: 15 print 'Always run' $ python catching.py Had an error: n should be an int Always run Friday, February 18, 2011

In the face of ambiguity, refuse the temptation to guess. • Coerce types only when not surprising • Cannot add strings and numbers • Can multiply them! Friday, February 18, 2011

Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str' Friday, February 18, 2011

Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str' Friday, February 18, 2011

Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str' Friday, February 18, 2011

Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str' Friday, February 18, 2011

Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str' Friday, February 18, 2011

There should be one – and preferably only one – way to do it. • Eliminate redundancy • Easier to learn • Easier to remember Friday, February 18, 2011

Index API 1 a = [ 1, 2, 3 ] 2 print a[0] 3 4 a = (1, 2, 3) 5 print a[0] 6 7 a = 'abc' 8 print a[0] 9 10 a = {0:'zero value', 11 1:'one value', 12 } 13 print a[0] $ python index_api.py 1 1 a zero value Friday, February 18, 2011

Although that way may not be obvious at first unless you’re Dutch. • GUI • GTK • KDE • Cocoa • MS Windows • Tk • wxPython • Web • Django • AppEngine • Pyramid • Flask • TurboGears • Pecan Friday, February 18, 2011

Now is better than never. • ctypes • Jython • IronPython • Python 2 vs. Python 3 Friday, February 18, 2011

Although never is often better than right now. • Language moratorium • Not everything goes into stdlib any more Friday, February 18, 2011

If the implementation is hard to explain, it’s a bad idea. Python Enhancement Proposals Friday, February 18, 2011

If the implementation is easy to explain, it may be a good idea. Friday, February 18, 2011

Namespaces are one honking great idea – let’s do more of those! Closures Friday, February 18, 2011

Closures 1 def outer(arg): 2 print 'outer(%s)' % arg 3 4 def inner(val): 5 print 'inner(%s) (arg=%s)' % (val, arg) 6 return val * arg 7 8 return inner 9 10 print 'Creating thrice:' 11 thrice = outer(3) 12 13 print '\nCalling thrice:' 14 print thrice(3) 15 print thrice('a') Friday, February 18, 2011

Closures 1 def outer(arg): 2 print 'outer(%s)' % arg 3 4 def inner(val): 5 print 'inner(%s) (arg=%s)' % (val, arg) 6 return val * arg 7 8 return inner 9 10 print 'Creating thrice:' 11 thrice = outer(3) 12 13 print '\nCalling thrice:' 14 print thrice(3) 15 print thrice('a') $ python closure.py Creating thrice: outer(3) Calling thrice: inner(3) (arg=3) 9 inner(a) (arg=3) aaa Friday, February 18, 2011

References • http://docs.python.org/ • http://www.doughellmann.com/ PyMOTW/ • Learning Python by Mark Lutz • Python Essential Reference by David Beazley • The Python Standard Library by Example by Doug Hellmann • http://us.pycon.org/ Friday, February 18, 2011

Questions? Friday, February 18, 2011

Image Credits 1. http://www.gaialandscapedesignbackgrounds.net/landscape_design_architecture_background_Zen_Garden_chan3.htm 3. http://www.flickr.com/photos/grabadonut/539151245/sizes/l/in/photostream/ 13. http://www.flickr.com/photos/tobascodagama/3106398602/ 17. http://www.flickr.com/photos/benandliz/11065336/ 21 & 22. http://scipy.org 24. http://www.otherlandtoys.co.uk/russian-matryoshka-nesting-dolls-pieces-p-2126.html 32. http://www.dandelionsummers.com/html/building_block.html 36. http://www.flickr.com/photos/photoinference/2494576240/in/faves-40068198@N04/ 47. http://www.flickr.com/photos/smithsonian/3112472619/in/faves-40068198@N04/ 54. http://www.flickr.com/photos/mherring/3722272868/sizes/l/in/photostream/ 57. http://www.webstaurantstore.com/9-3-4-x-4-1-2-unbleached-natural-coffee-filter-12-cup-1000-cs/121CF12U.html 59. http://brainstorm-services.com/wcu-lit/ambiguity.html 66. http://commons.wikimedia.org/wiki/File:Dutch_windmill.jpg 67. http://www.epicpm.org/epic-blogging 68. http://www.skyscanner.net/news/articles/2009/09/002990-travel-tip-10-get-on-last-and-learn-the-art-of-patience.html 69. http://gizmodo.com/5015735/the-top-10-rube-goldberg-machines-featured-on-film 71. http://www.nutricaplabs.com/capsule-manufacturer.aspx 75. http://wmltech.wordpress.com/2006/12/09/got-questions/ Friday, February 18, 2011