An Introduction to the Zen of Python

An introduction to the Python programming language organized around "The Zen of Python".

Transcript

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

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2. 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
   

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

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4. 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
   

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5. 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
   

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6. 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
   

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7. 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
   

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8. 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
   

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9. 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
   

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10. 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
    

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11. 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
    

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12. 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
    

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13. 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
    

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

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15. 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
    

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16. 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
    

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17. 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
    

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18. 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
    

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19. 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
    

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20. 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
    

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21. 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
    

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22. Simple is better than
    complex.
    • Garbage collection
    • print statement
    • Interactive interpreter for
    experimentation
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23. 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
    

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24. 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'}
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25. Interactive Interpreter
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26. Complex is better
    than complicated.
    • Do complex things in a
    “Pythonic” way
    • SciPy and NumPy for advanced
    science and math
    Friday, February 18, 2011
    

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

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28. 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”
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29. 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
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30. Standard Library Modules: Networking
    • socket
    • select
    • SocketServer
    • BaseHTTPServer
    • asyncore
    • asynchat
    • xmlrpclib
    • SimpleXMLRPCServer
    Friday, February 18, 2011
    

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

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

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

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

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

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36. Sparse is better than
    dense.
    • Standard library stays “shallow”
    • http://pypi.python.org/
    • Allows separate release
    schedules
    • pip (“pip installs Python
    packages”)
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37. Readability counts.
    • Whitespace for block structures
    • Minimal punctuation
    • Built-in documentation
    • Extensive online documentation
    at http://docs.python.org/
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38. 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
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39. 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
    

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40. 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
    

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41. 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
    

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42. 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
    

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43. 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
    

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44. 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
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45. 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
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46. 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
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47. 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
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48. 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
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49. 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
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50. 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
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51. 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
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52. 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
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53. 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
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54. 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
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55. 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
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56. Although practicality
    beats purity.
    • Multiple Programming Models
    • OOP
    • Procedural
    • Functional
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57. 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]
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58. Functional: List Comprehension
    1 results = [ i*i for i in range(5) ]
    2
    3 print results
    $ python functional.py
    [0, 1, 4, 9, 16]
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59. Functional: List Comprehension
    1 results = [ i*i for i in range(5) ]
    2
    3 print results
    $ python functional.py
    [0, 1, 4, 9, 16]
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60. Functional: List Comprehension
    1 results = [ i*i for i in range(5) ]
    2
    3 print results
    $ python functional.py
    [0, 1, 4, 9, 16]
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61. Functional: List Comprehension
    1 results = [ i*i for i in range(5) ]
    2
    3 print results
    $ python functional.py
    [0, 1, 4, 9, 16]
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62. 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
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63. 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
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64. Errors should never
    pass silently.
    • Exception-based error handling
    • Tracebacks aid debugging
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65. 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'
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66. 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
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67. 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
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68. Unless explicitly
    silenced.
    • Catch exceptions with
    try:except
    • Process
    • Convert
    • Ignore
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69. 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
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70. In the face of ambiguity, refuse the temptation to
    guess.
    • Coerce types only when not
    surprising
    • Cannot add strings and
    numbers
    • Can multiply them!
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71. 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'
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72. 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'
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73. 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'
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74. 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'
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75. 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'
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76. There should be one – and
    preferably only one – way to do
    it.
    • Eliminate redundancy
    • Easier to learn
    • Easier to remember
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77. 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
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78. 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
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79. Now is better than
    never.
    • ctypes
    • Jython
    • IronPython
    • Python 2 vs. Python 3
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80. Although never is often
    better than right now.
    • Language moratorium
    • Not everything goes into stdlib
    any more
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81. If the implementation is hard
    to explain, it’s a bad idea. Python Enhancement Proposals
    Friday, February 18, 2011
    

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

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83. Namespaces are one
    honking great idea –
    let’s do more of
    those!
    Closures
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84. 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')
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85. 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
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86. 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/
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87. Questions?
    Friday, February 18, 2011
    

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88. 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/
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