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Python Tips, Tricks,
and Hidden Features
(using type, collections, meta-classes, and more)
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I'm Dustin
http://github.com/di
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No content
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Irked.
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Irked.
>>> j = object()
>>> j.hi = 'there'
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'object' object has no attribute 'hi'
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type
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type
>>> type(['foo', 'bar'])
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type
>>> type(['foo', 'bar'])
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type
>>> type(list)
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type
>>> type(list)
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type
>>> type(type)
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type
>>> type(type)
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type
>>> type(None)
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type
>>> type(None)
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type
>>> def func():
... pass
...
>>> type(func)
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type
>>> def func():
... pass
...
>>> type(func)
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type
>>> import types
>>> dir(types)
['BooleanType', 'BufferType', 'BuiltinFunctionType', 'BuiltinMethodType',
'ClassType', 'CodeType', 'ComplexType', 'DictProxyType', 'DictType',
'DictionaryType', 'EllipsisType', 'FileType', 'FloatType', 'FrameType',
'FunctionType', 'GeneratorType', 'GetSetDescriptorType', 'InstanceType',
'IntType', 'LambdaType', 'ListType', 'LongType', 'MemberDescriptorType',
'MethodType', 'ModuleType', 'NoneType', 'NotImplementedType', 'ObjectType',
'SliceType', 'StringType', 'StringTypes', 'TracebackType', 'TupleType',
'TypeType', 'UnboundMethodType', 'UnicodeType', 'XRangeType', '__all__',
'__builtins__', '__doc__', '__file__', '__name__', '__package__']
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type
>>> import types
>>> type(types)
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type
>>> import types
>>> type(types)
>>> type(types) == types.ModuleType
True
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type
>>> class FooClass:
... pass
...
>>> type(FooClass)
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type
>>> class FooClass: # Python 2.7
... pass
...
>>> type(FooClass)
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type
>>> class FooClass: # Python 3.4
... pass
...
>>> type(FooClass)
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type
>>> class FooClass(object): # Python 2.7
... pass
...
>>> type(FooClass)
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type
>>> type(42) is int
True
>>> type(42)()
0
>>> int()
0
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type
>>> j = type()
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type
>>> j = type()
Traceback (most recent call last):
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TypeError: type() takes 1 or 3 arguments
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type
>>> j = type('FooClass', (object,), {'hi': 'there'})
>>> type(j)
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type
>>> j = type('FooClass', (object,), {'hi': 'there'})
>>> j.hi
'there'
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type
>>> j = type('', (object,), {'hi': 'there'})
>>> j.hi
'there'
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type
>>> j = type('', (), {'hi': 'there'})
>>> j.hi
'there'
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type
>>> j = type('', (), {})
>>> j.hi = 'there'
>>> j.hi
'there'
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type
>>> j = object()
>>> j.hi = 'there'
Traceback (most recent call last):
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AttributeError: 'object' object has no attribute 'hi'
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type
>>> class FooClass():
... pass
...
>>> hasattr(FooClass(), '__dict__')
True
>>> hasattr(object(), '__dict__')
False
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type
>>> from stackoverflow import getsize¹
>>> getsize(object())
16
>>> getsize(0)
24
>>> getsize(dict())
280
>>> getsize(FooClass())
344
1 http://stackoverflow.com/a/30316760/4842627
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Questions?
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collections
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collections
>>> def group(names):
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... d = {}
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... d = {}
... for name in names:
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... d = {}
... for name in names:
... key = len(name)
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... d = {}
... for name in names:
... key = len(name)
... if key not in d:
... d[key] = []
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... d = {}
... for name in names:
... key = len(name)
... if key not in d:
... d[key] = []
... d[key].append(name)
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... d = {}
... for name in names:
... key = len(name)
... if key not in d:
... d[key] = []
... d[key].append(name)
... return d
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... d = {}
... for name in names:
... key = len(name)
... d.setdefault(key, []).append(name)
... return d
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections
>>> def group(names):
... return {
... length: [
... name for name in names if len(name) == length
... ]
... for length in {len(name) for name in names}
... }
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections.defaultdict
>>> from collections import defaultdict
>>> def group(names):
... d = defaultdict(list)
... for name in names:
... key = len(name)
... d[key].append(name)
... return dict(d)
...
>>> group(['tom', 'dick', 'harry', 'guido'])
{3: ['tom'], 4: ['dick'], 5: ['harry', 'guido']}
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collections.defaultdict
trie: an ordered tree data structure
that is used to store a dynamic set or
associative array where the keys are
usually strings2.
e.g.: "peter piper picked
a peck of pickled peppers"
2 https://en.wikipedia.org/wiki/Trie
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collections.defaultdict
>>> trie = ...
>>> trie['b']['a']['r'] = True
>>> trie['b']['a']['r'] == True
True
>>> trie['f']['o']['o'] == True
False
>>> trie
{'b': {'a': {'r': True}}}
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collections.defaultdict
>>> trie = {}
>>> trie['b']['a']['r'] = True
Traceback (most recent call last):
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KeyError: 'b'
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collections.defaultdict
>>> from collections import defaultdict
>>> trie = defaultdict(lambda: defaultdict(dict))
>>> trie['b']['a']['r'] = True
>>> trie['b']['a']['r'] == True
True
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collections.defaultdict
>>> from collections import defaultdict
>>> trie = defaultdict(lambda: defaultdict(dict))
>>> trie['b']['a']['r'] = True
>>> trie['b']['a']['r'] == True
True
>>> trie['f']['o']['o'] == True
Traceback (most recent call last):
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KeyError: 'o'
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collections.defaultdict
>>> from collections import defaultdict
>>> trie = defaultdict(lambda: defaultdict(dict))
>>> trie['t']['r']['i']['e'] = True
Traceback (most recent call last):
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KeyError: 'i'
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collections.defaultdict
>>> from collections import defaultdict
>>> trie = defaultdict(lambda: defaultdict(
... lambda: defaultdict(lambda: defaultdict(
... lambda: defaultdict(lambda: defaultdict(
... dict))))))
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collections.defaultdict
>>> from collections import defaultdict
>>> infinitedict = lambda: defaultdict(infinitedict)
>>> trie = infinitedict()
>>> trie['b']['a']['r'] = True
>>> trie['b']['a']['r'] == True
True
>>> trie['f']['o']['o'] == True
False
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collections.defaultdict
>>> from collections import defaultdict
>>> def infinitedict():
... return defaultdict(infinitedict)
...
>>> trie = infinitedict()
>>> trie['b']['a']['r'] = True
>>> trie['b']['a']['r'] == True
True
>>> trie['f']['o']['o'] == True
False
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collections
>>> def count(names):
...
>>> count(['beetlejuice', 'beetlejuice', 'guido'])
{'beetlejuice': 2, 'guido': 1}
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collections
>>> from collections import defaultdict
>>> def count(names):
... d = defaultdict(int)
...
>>> count(['beetlejuice', 'beetlejuice', 'guido'])
{'beetlejuice': 2, 'guido': 1}
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collections
>>> from collections import defaultdict
>>> def count(names):
... d = defaultdict(int)
... for name in names:
... d[name] += 1
...
>>> count(['beetlejuice', 'beetlejuice', 'guido'])
{'beetlejuice': 2, 'guido': 1}
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collections
>>> from collections import defaultdict
>>> def count(names):
... d = defaultdict(int)
... for name in names:
... d[name] += 1
... return dict(d)
...
>>> count(['beetlejuice', 'beetlejuice', 'guido'])
{'beetlejuice': 2, 'guido': 1}
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collections.Counter
>>> from collections import Counter
>>> def count(names):
... return dict(Counter(names))
...
>>> count(['beetlejuice', 'beetlejuice', 'guido'])
{'beetlejuice': 2, 'guido': 1}
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Questions?
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Metaclasses
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Metaclasses
>>> class FooClass():
... pass
...
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Metaclasses
>>> class FooClass():
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
False
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Metaclasses
>>> class Singleton():
...
>>> class FooClass(Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton():
... _instance = None
...
>>> class FooClass(Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton():
... _instance = None
... def __new__(cls, *args, **kwargs):
...
>>> class FooClass(Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton():
... _instance = None
... def __new__(cls, *args, **kwargs):
... if not cls._instance:
...
>>> class FooClass(Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton():
... _instance = None
... def __new__(cls, *args, **kwargs):
... if not cls._instance:
... cls._instance = object.__new__(cls, *args, **kwargs)
...
>>> class FooClass(Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton():
... _instance = None
... def __new__(cls, *args, **kwargs):
... if not cls._instance:
... cls._instance = object.__new__(cls, *args, **kwargs)
... return cls._instance
...
>>> class FooClass(Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
classes : instances : : metaclasses : classes
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Metaclasses
>>> j = type('FooClass', (object,), {'hi': 'there'})
>>> type(j)
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Metaclasses
>>> j = type('FooClass', (object,), {'hi': 'there'})
>>> type(j)
>>> type(j())
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Metaclasses
>>> class MyMeta(type):
... pass
...
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Metaclasses
>>> class MyMeta(type):
... pass
...
>>> class FooClass(metaclass=MyMeta):
... pass
...
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Metaclasses
>>> class MyMeta(type):
... pass
...
>>> class FooClass(object): # Python 2.7
... __metaclass__ = MyMeta
...
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Metaclasses
>>> class MyMeta(type):
... def __new__(meta, name, bases, attrs):
... return super().__new__(meta, name, bases, attrs)
...
>>> class FooClass(metaclass=MyMeta):
... pass
...
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Metaclasses
>>> class MyMeta(type):
... def __new__(meta, name, bases, attrs):
... print('New {}'.format(name))
... return super().__new__(meta, name, bases, attrs)
...
>>> class FooClass(metaclass=MyMeta):
... pass
...
New FooClass
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Metaclasses
>>> class MyMeta(type):
... def __call__(cls, *args, **kwargs):
... print('Call {}'.format(cls.__name__))
... return super().__call__(*args, **kwargs)
...
>>> class FooClass(metaclass=MyMeta):
... pass
...
>>> f = FooClass()
Call FooClass
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Metaclasses
>>> class Singleton(type):
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton(type):
... def __new__(meta, name, bases, attrs):
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton(type):
... def __new__(meta, name, bases, attrs):
... attrs['_instance'] = None
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton(type):
... def __new__(meta, name, bases, attrs):
... attrs['_instance'] = None
... return super().__new__(meta, name, bases, attrs)
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton(type):
... def __new__(meta, name, bases, attrs):
... attrs['_instance'] = None
... return super().__new__(meta, name, bases, attrs)
... def __call__(cls, *args, **kwargs):
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton(type):
... def __new__(meta, name, bases, attrs):
... attrs['_instance'] = None
... return super().__new__(meta, name, bases, attrs)
... def __call__(cls, *args, **kwargs):
... if not cls._instance:
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton(type):
... def __new__(meta, name, bases, attrs):
... attrs['_instance'] = None
... return super().__new__(meta, name, bases, attrs)
... def __call__(cls, *args, **kwargs):
... if not cls._instance:
... cls._instance = super().__call__(*args, **kwargs)
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Metaclasses
>>> class Singleton(type):
... def __new__(meta, name, bases, attrs):
... attrs['_instance'] = None
... return super().__new__(meta, name, bases, attrs)
... def __call__(cls, *args, **kwargs):
... if not cls._instance:
... cls._instance = super().__call__(*args, **kwargs)
... return cls._instance
...
>>> class FooClass(metaclass=Singleton):
... pass
...
>>> a, b = FooClass(), FooClass()
>>> a is b
True
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Questions?
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wat
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wat #0
>>> x = ...
>>> x == x
False
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wat #0 - Possible!
>>> x = float('nan')
>>> x == x
False
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wat #0 - Possible!
>>> x = 0*1e400
>>> x == x
False
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http://python-wats.herokuapp.com/
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wat #1
>>> x = ...
>>> a = ...
>>> b = ...
>>> c = ...
>>> max(x) < max(x[a:b:c])
True
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wat #1 - Not Possible
>>> x = ...
>>> a = ...
>>> b = ...
>>> c = ...
>>> max(x) < max(x[a:b:c])
True
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wat #2
>>> x = ...
>>> y = ...
>>> min(x, y) == min(y, x)
False
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wat #2 - Possible!
>>> x = {0}
>>> y = {1}
>>> min(x, y) == min(y, x)
False
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wat #2 - Possible!
>>> min({0}, {1})
set([0])
>>> min({1}, {0})
set([1])
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wat #2 - Possible!
>>> min({0}, {1})
set([0])
>>> min({1}, {0})
set([1])
>>> min({0, 1}, {0})
set([0])
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wat #2 - Possible!
>>> def min(*args):
...
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wat #2 - Possible!
>>> def min(*args):
... has_item = False
... min_item = None
...
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wat #2 - Possible!
>>> def min(*args):
... has_item = False
... min_item = None
... for x in args:
...
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wat #2 - Possible!
>>> def min(*args):
... has_item = False
... min_item = None
... for x in args:
... if not has_item or x < min_item:
...
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wat #2 - Possible!
>>> def min(*args):
... has_item = False
... min_item = None
... for x in args:
... if not has_item or x < min_item:
... has_item = True
... min_item = x
...
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wat #2 - Possible!
>>> def min(*args):
... has_item = False
... min_item = None
... for x in args:
... if not has_item or x < min_item:
... has_item = True
... min_item = x
... return min_item
...
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wat #2 - Possible!
>>> {1} < {0}
False
>>> {1} < {0, 1}
True
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wat #2 - Possible!
>>> {1} < {0}
False
>>> {1} < {0, 1}
True
>>> min({0}, {1})
set([0])
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wat #3
>>> x = ...
>>> y = ...
>>> any(x) and not any(x + y)
True
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wat #3 - Not Possible
>>> x = ...
>>> y = ...
>>> any(x) and not any(x + y)
True
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wat #4
>>> x = ...
>>> y = ...
>>> x.count(y) > len(x)
True
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wat #4 - Possible!
>>> x = 'a'
>>> y = ''
>>> x.count(y) > len(x)
True
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wat #4 - Possible!
>>> x = 'a'
>>> y = ''
>>> x.count(y) > len(x)
True
>>> len('a')
1
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wat #4 - Possible!
>>> x = 'a'
>>> y = ''
>>> x.count(y) > len(x)
True
>>> len('a')
1
>>> 'a'.count('')
2
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wat #4 - Possible!
>>> def count(s, sub):
...
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wat #4 - Possible!
>>> def count(s, sub):
... result = 0
...
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wat #4 - Possible!
>>> def count(s, sub):
... result = 0
... for i in range(len(s) + 1 - len(sub)):
...
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wat #4 - Possible!
>>> def count(s, sub):
... result = 0
... for i in range(len(s) + 1 - len(sub)):
... possible_match = s[i:i + len(sub)]
...
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wat #4 - Possible!
>>> def count(s, sub):
... result = 0
... for i in range(len(s) + 1 - len(sub)):
... possible_match = s[i:i + len(sub)]
... if possible_match == sub:
... result += 1
... return result
...
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wat #4 - Possible!
>>> def count(s, sub):
... result = 0
... for i in range(len(s) + 1 - len(sub)):
... possible_match = s[i:i + len(sub)]
... if possible_match == sub:
... result += 1
... return result
...
>>> count('a', '')
2
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wat #4 - Possible!
>>> count('foofoof', 'foof') # my implementation
2
>>> 'foofoof'.count('foof')
1
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wat #5
>>> x = ...
>>> y = ...
>>> z = ...
>>> x * (y * z) == (x * y) * z
False
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wat #5 - Possible!
>>> x = [0]
>>> y = -1
>>> z = -1
>>> x * (y * z) == (x * y) * z
False
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wat #5 - Possible!
>>> x = [0]
>>> y = -1
>>> z = -1
>>> x * (y * z) == (x * y) * z
False
>>> x * (y * z) == [0]*(-1*-1)
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wat #5 - Possible!
>>> x = [0]
>>> y = -1
>>> z = -1
>>> x * (y * z) == (x * y) * z
False
>>> x * (y * z) == [0]*(-1*-1) == [0]*1
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wat #5 - Possible!
>>> x = [0]
>>> y = -1
>>> z = -1
>>> x * (y * z) == (x * y) * z
False
>>> x * (y * z) == [0]*(-1*-1) == [0]*1 == [0]
True
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wat #5 - Possible!
>>> x = [0]
>>> y = -1
>>> z = -1
>>> x * (y * z) == (x * y) * z
False
>>> x * (y * z) == [0]*(-1*-1) == [0]*1 == [0]
True
>>> (x * y) * z == ([0]*-1)*-1
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wat #5 - Possible!
>>> x = [0]
>>> y = -1
>>> z = -1
>>> x * (y * z) == (x * y) * z
False
>>> x * (y * z) == [0]*(-1*-1) == [0]*1 == [0]
True
>>> (x * y) * z == ([0]*-1)*-1 == []*-1
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wat #5 - Possible!
>>> x = [0]
>>> y = -1
>>> z = -1
>>> x * (y * z) == (x * y) * z
False
>>> x * (y * z) == [0]*(-1*-1) == [0]*1 == [0]
True
>>> (x * y) * z == ([0]*-1)*-1 == []*-1 == []
True
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wat #6
>>> x = ...
>>> y = ...
>>> x < y and all(a >= b for a, b in zip(x, y))
True
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wat #6 - Possible!
>>> x = []
>>> y = [0]
>>> x < y and all(a >= b for a, b in zip(x, y))
True
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wat #6 - Possible!
>>> x = []
>>> y = [0]
>>> x < y and all(a >= b for a, b in zip(x, y))
True
>>> [] < [0]
True
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wat #6 - Possible!
>>> x = []
>>> y = [0]
>>> x < y and all(a >= b for a, b in zip(x, y))
True
>>> [] < [0]
True
>>> zip([], [0])
[]
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wat #6 - Possible!
>>> x = []
>>> y = [0]
>>> x < y and all(a >= b for a, b in zip(x, y))
True
>>> [] < [0]
True
>>> zip([], [0])
[]
>>> all([])
True
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wat #7
>>> x = ...
>>> len(set(list(x))) == len(list(set(x)))
False
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wat #7 - Not Possible
>>> x = ...
>>> len(set(list(x))) == len(list(set(x)))
False
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wat #8
>>> x = ...
>>> min(x) == min(*x)
False
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wat #8 - Possible!
>>> x = [[0]]
>>> min(x) == min(*x)
False
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wat #8 - Possible!
>>> x = [[0]]
>>> min(x) == min(*x)
False
>>> min([1, 2, 3]) == min(*[1, 2, 3]) == min(1, 2, 3)
True
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wat #8 - Possible!
>>> x = [[0]]
>>> min(x) == min(*x)
False
>>> min([1, 2, 3]) == min(*[1, 2, 3]) == min(1, 2, 3)
True
>>> min(x) == [0]
True
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wat #8 - Possible!
>>> x = [[0]]
>>> min(x) == min(*x)
False
>>> min([1, 2, 3]) == min(*[1, 2, 3]) == min(1, 2, 3)
True
>>> min(x) == [0]
True
>>> min(*x) == min([0]) == 0
True
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wat #9
>>> x = ...
>>> y = ...
>>> sum(0 * x, y) == y
False
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wat #9 - Not Possible
>>> x = ...
>>> y = ...
>>> sum(0 * x, y) == y
False
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wat #9 - Not Possible
>>> x = ...
>>> y = ...
>>> sum(0 * x, y) == y
False
>>> sum([1, 1, 1], 7)
10
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wat #9 - Not Possible
>>> x = ...
>>> y = ...
>>> sum(0 * x, y) == y
False
>>> sum([1, 1, 1], 7)
10
>>> sum([], 7)
7
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wat #10
>>> x = ...
>>> y = ...
>>> y > max(x) and y in x
True
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wat #10 - Possible!
>>> x = 'aa'
>>> y = 'aa'
>>> y > max(x) and y in x
True
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wat #10 - Possible!
>>> x = 'aa'
>>> y = 'aa'
>>> y > max(x) and y in x
True
>>> max('aa')
'a'
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wat #10 - Possible!
>>> x = 'aa'
>>> y = 'aa'
>>> y > max(x) and y in x
True
>>> max('aa')
'a'
>>> 'aa' > 'a'
True
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wat #10 - Possible!
>>> x = 'aa'
>>> y = 'aa'
>>> y > max(x) and y in x
True
>>> max('aa')
'a'
>>> 'aa' > 'a'
True
>>> 'aa' in 'aa'
True
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Questions?
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Thanks!