Some python notes for pre-interview revision

Range vs Xrange

xrange is a sequence object that evaluates lazily. range creates a list, so if you do range(1, 10000000) it creates a list in memory with 10000000 elements. xrange is a generator, so it is a sequence object is a that evaluates lazily.

Tuple

Immutable
s = ("a","b","c") #paranthesis are optional
Indexing support just like lists
print(1,2,3) #prints 3 numbers
print((1,2,3)) #prints a tuple
a = () #empty tuple
a = (2,) #singleton tuple
a,b = interprets the result of the expression as a tuple with two values.
a,b = b,a #swaps the variables
a,b = [1,2] #obvious
a,b = [1,2,3] #ValueError : Too many values to unpack
a,*b = [1,2,3] #a=1 and b=[2,3]

Dictionary

A hash map, key value pairs, is unordered.
Keys must be unique and only consist of immutable objects like strings

Sequences

Lists, strings and tuples are example of sequences.
Supports "Membership Tests" : (for i in arr, for i not in arr)
Supports Indexing Operations and Slicing

Set

Unordered collection of simple objects
Supports test for membership

References

arr = [1,2,3]
arr2 = arr
arr3 = arr[:] #this makes a copy by full slicing
del arr[0]
print arr2 #[2,3]
print arr3 #[1,2,3]
Explaination: Assigning a variabel name to another variable name just makes creates a reference to the original object. If you want to make a completely new object and not just a reference then you have to use copy.

Object Oriented Programming

Definitions

Objects that belong to the class or the instance of the class are called fields.
Objects that belong to the class are called class variables and objects that belong to the instance of the class are called instance variables.

Self

Python equivalent of this pointer in C++ or this reference in java and C#.
The name self can be changed to something else also, although its not recommended you do that.
Self is used to create a reference to the object being created of the class, so a call like 'myobject.method(arg1,arg2)' gets internally inflated as 'MyClass.method(myobject,arg1,arg2)'

@staticmethod decorator

Those methods which belong exclusively to the class and not its instances

Implementing Inheritance

class SchoolMember():...
class Teacher(SchoolMember):...

Special Methods in Classes

__inti__(self, ...) : Called right before the newly created object is returned for usage.
__del__(self) : Called just before the object is destroyed.
__str__(self) : Called when we use the print function of when str() is used.
__lt__(self,other) : called when less than operator is used (<), Similarly there are methods for all the operators (+,>,- etc)
__getitem__(self,key): Called when x[key] indexing call is made.
__len__(self) : Called when the built-in len() function is used for sequence object.

Exceptions

Else: this block is executed in the case everything is fine and no exceptions occur.
Finally: this block is executed when all the exception cases are met and the interpreter is about to exit the try block

User defined Exception Class

class ShortInputException():
    def __init__(self,length,atleast):
        Exception.__init__(self):
        self.length = length
        self.atleast = atleast


#Now lets raise this exception
if len(text) < 3: raise ShortInputException(len(text),3)

Lambda Expression

A lambda statement is used to create new function objects.

points = [{'x':2,'y':30},{'x':1,'y':9}] 
points.sort(key=lambda i : i['y']) 
[{’x’: 1, ’y’: 9}, {’x’: 2, ’y’: 30}]

Recieving tuples and dictionaries of indefinite lengths in functions

oints.sort(key=lambda i : i['y']) 
def sum(*args): 
    ''' Returns the sume of all the parameters ''' 
    return sum(a)




sum(1,43,54,7,8)




>>> 113

Because we have a * prefix on the args variable, all extra arguments passed to the function are stored in args as a tuple. If a ** prefix had been used instead, the extra parameters would be considered to be key/value pairs of a dictionary.

The Assert Statement


def sum(a,b): 
    assert type(a) == int 
    assert type(b) == int 
    return sum(a,b)

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