The Essence of Python
Objects, Values, Types, and Bindings
Everything is an object.
Every object has
a value.
a type.
an identity.
a namespace.
CS105 Python
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Everything is an object.
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value
1
value
2
Every object has a value.
value -0.33
CS105 Python
value 100.0
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Numbers
Name (type)
Integer (int)
Floating-point (float)
Values
...,-1, 0, 1, ...
-1.3, 100.777
Operations
arithmetic: +, -, *, **, /, //, %
comparison: ==, !=, >, >=, <, <=
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Spring 2009
value
1
value
type int
2
type int
Every object has a type.
value -0.33
type float
CS105 Python
value 100.0
type float
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Inspecting Types
>>> type(1)
<type 'int'>
>>>
>>> type(1.0)
<type 'float'>
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Truth
Name (type)
Boolean (bool)
Values
True, False
Operations
CS105 Python
logical: and, or, not
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Bindings
global namespace
Python 2.6.1 ...
Type "help", ...
>>>
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Bindings
global namespace
x
CS105 Python
Python 2.6.1 ...
Type "help", ...
>>>
>>> x = 1
1
type
int
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Bindings
global namespace
Python 2.6.1 ...
Type "help", ...
>>>
>>> x = 1
>>>
>>> x = True
x
True
type bool
CS105 Python
X
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Strings
Name (type)
String (str)
Values
'hi',
"hi",
'''Hello''',
"""That's all,
folks"""
Operations
CS105 Python
construction +, *
comparison: ==, !=, >, >=, <, <=
membership: in
built-in: len(str), and more...
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Spring 2009
Sequences
Name (type)
List (list)
Values
[], [1], ['a', 'b']
Operations
CS105 Python
access: list[index]
construction +, *
comparison: ==, !=, >, >=, <, <=
membership: in
built-in: len(list), and more...
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Slicing A Sequence
seq[start : end : step]
0 1
10
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
-10
-2
-1
think: [start, end)
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value
1
type
int
id 8405384
value
'a'
type
str
id 269440
Every object has an identity.
value [1,2,3]
type
list
id 4161168
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Spring 2009
Inspecting Identities
>>> x = [1,2,3]
>>> id(x)
4160448
>>>
>>> y = x
>>> id(y)
4160448
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Identity and Equality
(== vs is)
>>> x
>>> y
>>> y
True
>>> y
True
>>>
>>> y
>>> y
True
>>> y
False
== compares values.
is compares identities.
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= [1,2,3]
= x
== x
is x
= [1,2,3]
== x
is x
Spring 2009
More Sequences
Name (type)
Tuple (tuple)
Values
(), (1,), ('Williams', 1941, .406)
Operations
CS105 Python
access: tuple[index]
construction +, *
comparison: ==, !=, >, >=, <, <=
membership: in
built-in: len(tuple), and more...
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Usage Patterns
Use list as a collection
Use tuple as a record
•Values have same type
•Values may have different types
•Values change over time
•Values constant over time
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Mutability
The value of a mutable type may change.
The value of an immutable type cannot change.
Mutable types
Immutable types
int
float
str
tuple
...
list
...
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Mapping
Name (type)
Dictionary (dict)
Values
{}, {1:'one'}, {1:'one', 2:'two'}
Operations
CS105 Python
Mu
access: dict[key]
imm st be
utab
an
l
e
removal: del dict[key]
valu
e
key membership: in
dict.keys(), dict.items(),
built-in:
dict.values(), and more...
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value
type
id
'a'
str
269440
name
value
namespace 'upper'
...
...
...
Every object has a namespace.
value
type
id
namespace
CS105 Python
{}
dict
515072
name
value
'keys'
...
...
...
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Spring 2009
Inspecting Namespaces
>>> dir({})
['__class__', '__cmp__', '__contains__', '__delattr__',
'__delitem__', '__doc__', '__eq__', '__format__',
'__ge__', '__getattribute__', '__getitem__', '__gt__',
'__hash__', '__init__', '__iter__', '__le__', '__len__',
'__lt__', '__ne__', '__new__', '__reduce__',
'__reduce_ex__', '__repr__', '__setattr__',
'__setitem__', '__sizeof__', '__str__',
'__subclasshook__', 'clear', 'copy', 'fromkeys', 'get',
'has_key', 'items', 'iteritems', 'iterkeys',
'itervalues', 'keys', 'pop', 'popitem', 'setdefault',
'update', 'values']
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Everything is an object.
Every object has
a value.
a type.
an identity.
a namespace.
CS105 Python
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Spring 2009