Advanced Aircraft Design Lab

Royal Military College of Canada

Python Sneaky Snakes¶

Integer Division¶

When python divides two integers the results is truncated to an integer:

>>> a = 1/2
0

>>> a = 1 / 16
0

>>> a = 15 / 16
0

>>> a = 1.0 / 2
0.5

List Indexing¶

If you slice a list beyond its bounds, it will not raise an error:

>>> a = [1, 2, 3, 4]
[1,2,3,4]

>>> a[0:3]
[1,2,3]

>>> a[2:4]
[3,4]

>>> a[2:5]
[3,4]

>>> a[2:500]
[3,4]

>>> a[5:6]
[]

You can use this to check if an index exists:

>>> isinstance(a, list)
True

>>> len(a)
4

>>> n = 5
>>> not a[n:(n+1)] == []
False

>>> n = 2
>>> not a[n:(n+1)] == []
True

Copies and Deep Copies¶

All python objects are pointers. Unless you make a copy of an object, when you assign one objet the value of another you are only pointing to it so any changes to one object affect the other:

>>> dict1 = {'a': 'apple', 'b':'bear'}
>>> dict2 = dict1

>>> print repr(dict1)
{'a': 'apple', 'b':'bear'}
>>> print repr(dict2)
{'a': 'apple', 'b':'bear'}

>>> dict2['a'] = 'antelope'
>>> print dict1['a']
'antelope'
# Expected 'apple'

>>> dict2 = copy.copy(dict1)
>>> dict2['a'] = 'aardvark'

>>> print repr(dict1)
{'a': 'antelope', 'b':'bear'}

>>> print repr(dict2)
{'a': 'aardvark', 'b':'bear'}

But copy only makes copies of the first level of objects. i.e in a dictionary of dictionaries only the first level of the dictionary is copied. To make a copy of all the objects you must make a deep copy. This should only be used when needed since there are some disadvantages which are explained on the python website:

>>> dict1 =
{       'a': {'fruit':'apple', 'animal':'aardvark'},
        'b': {'fruit':'banana', 'animal':'bear'},}

>>> dict2 = copy.copy(dict1)
>>> dict2['a']['anaimal'] = 'antelope'

>>> print dict1['a']['animal']
'antelope'
# Expected aardvark

>>> dict2 = copy.deepcopy(dict1)
>>> dict2['b']['fruit'] = 'blueberry'

>>> print repr(dict1)
{       'a': {'fruit':'apple', 'animal':'antelope'},
        'b': {'fruit':'banana', 'animal':'bear'},}

>>> print repr(dict2)
{       'a': {'fruit':'apple', 'animal':'antelope'},
        'b': {'fruit':'blueberry', 'animal':'bear'},}

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