python：power(a,b)、list.Slice、list.reverse、try-except、object-oriented programming (OOP)

>>> 2**10

  
  
  
  
1024

equal to math.power(2, 10)

Slicing syntax: L[start:stop:stride]

 

All slicing parameters are optional:

>>>

>>> L

     
     
     
     
['red', 'blue', 'green', 'black', 'white']

     
     
     
     
>>> L[3:]

     
     
     
     
['black', 'white']

     
     
     
     
>>> L[:3]

     
     
     
     
['red', 'blue', 'green']

     
     
     
     
>>> L[::2]

     
     
     
     
['red', 'green', 'white']

Reverse:

>>>

>>> r = L[::-1]

    
    
    
    
>>> r

    
    
    
    
['white', 'black', 'green', 'blue', 'red']

    
    
    
    
>>> r2 = list(L)

    
    
    
    
>>> r2

    
    
    
    
['red', 'blue', 'green', 'black', 'white']

    
    
    
    
>>> r2.reverse() # in-place

    
    
    
    
>>> r2

    
    
    
    
['white', 'black', 'green', 'blue', 'red']

def print_sorted(collection):

  
  
  
  
   ....:     try:

  
  
  
  
   ....:         collection.sort()

  
  
  
  
   ....:     except AttributeError:

  
  
  
  
   ....:         pass

  
  
  
  
   ....:     print(collection)

def filter_name(name):

  
  
  
  
   ....:    try:

  
  
  
  
   ....:        name = name.encode('ascii')

  
  
  
  
   ....:    except UnicodeError, e:

  
  
  
  
   ....:        if name == 'Gaël':

  
  
  
  
   ....:            print('OK, Gaël')

  
  
  
  
   ....:        else:

  
  
  
  
   ....:            raise e

  
  
  
  
   ....:    return name

>>> class Student(object):

    
    
    
    
...     def __init__(self, name):

    
    
    
    
...         self.name = name

    
    
    
    
...     def set_age(self, age):

    
    
    
    
...         self.age = age

    
    
    
    
...     def set_major(self, major):

    
    
    
    
...         self.major = major

    
    
    
    
...

    
    
    
    
>>> anna = Student('anna')

    
    
    
    
>>> anna.set_age(21)

    
    
    
    
>>> anna.set_major('physics')

    
    
    
    

In the previous example, the Student class has __init__, set_age and set_majormethods. Its attributes are name, age and major. We can call these methods and attributes with the following notation: classinstance.method or classinstance.attribute. The __init__ constructor is a special method we call with: MyClass(init parameters ifany).

Now, suppose we want to create a new class MasterStudent with the same methods and attributes as the previous one, but with an additional internship attribute. We won’t copy the previous class, but inherit from it:

>>>

>>> class MasterStudent(Student):

    
    
    
    
...     internship = 'mandatory, from March to June'

    
    
    
    
...

    
    
    
    
>>> james = MasterStudent('james')

    
    
    
    
>>> james.internship

    
    
    
    
'mandatory, from March to June'

    
    
    
    
>>> james.set_age(23)

    
    
    
    
>>> james.age

    
    
    
    
23

    
    
    
    

The MasterStudent class inherited from the Student attributes and methods.

Thanks to classes and object-oriented programming, we can organize code with different classes corresponding to different objects we encounter (an Experiment class, an Image class, a Flow class, etc.), with their own methods and attributes. Then we can use inheritance to consider variations around a base class and re-use code. Ex : from a Flow base class, we can create derived StokesFlow, TurbulentFlow, PotentialFlow, etc.