Some words on equality

A variable in python stores the reference to a register in the memory, not the actual value. Each variable has a unique id.



In [1]:

    
a = 5.
print(id(a))
b = 5.
print(id(b))

If you assign a variable, the ID is passed. This means that the same object has just only two different names, but it is still the same object. This saves memory.



In [5]:

    
c = a
print id(c)
print c is a
print c == a
print b == a
print b is c









    



42661648
True
True
True
False



In [6]:

    
print(c)
print c is a
c = 4.
print(c)
print(a)
print c is a









    



5.0
True
4.0
5.0
False



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# let's do some changes
c += 2.
print(c)
print c is a



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print c
c -= 2.
print(c, a)
print c is a
print c == a

Remember

The is function checks if the ID of two objects are the same.
The = operator checks if the numerical value is the same

How is it for strings?



In [7]:

    
s1 = 'hello'
s2 = 'world'
print(s1 is s2)
print(s1 == s2)









    



False
False



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s3 = s1
print s1 is s3



In [8]:

    
s4 = 'hello'  
print(s1 is s4)   # note that even a new variable is generated, while its contents already exists, just a pointer is used
print(s1 == s4)
print(id(s1),id(s4))









    



True
True
(140406335508912, 140406335508912)



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s4 += ' geography'
print(s4)
print(id(s4))
print(s4 is s1)

Always be aware that an assignment might be just a pointer!



In [16]:

    
import numpy as np # we talk about numpy later today
A = np.array([[1, 2], [3, 4]])
A









    Out[16]:





array([[1, 2],
       [3, 4]])



In [17]:

    
B=A*1.   # copy, deepcopy
B









    Out[17]:





array([[ 1.,  2.],
       [ 3.,  4.]])



In [18]:

    
print id(A), id(B)









    



140406239478528 140406239477888



In [19]:

    
B[0,0]=10.
B









    Out[19]:





array([[ 10.,   2.],
       [  3.,   4.]])

Note what happened to A now ...



In [20]:

    
A









    Out[20]:





array([[1, 2],
       [3, 4]])



In [14]:

    
C = A
C[1,1] = 77.



In [15]:

    
B









    Out[15]:





array([[10,  2],
       [ 3, 77]])

Where does it matter in particular? Working with functions



In [ ]:

    
def foo(a,b):
    print(a + ' ' + b)
    
a = 'Hello'
b = 'World'
foo(a,b)



In [ ]:

    
def foo1(a,b):
    c = a+b
    a = 'Munich'
    print(c)
    
foo1(a,b)
print(a)  # the original variable was unchanged

Remember: Variables have a scope; e.g. within a functionmm



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