Python Programming: Useful Tips and Tricks

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In this post, we will learn to use some basic tips and tricks of Python.

  1. Swap Two Numbers
     >>> x, y = 5, 10
     >>> x, y = y, x
     >>> x
     10
     >>> y
     5
    
  2. Reversing String
     >>> x = "hello"
     >>> x[::-1]
     'olleh'
    
  3. Sort a list
     >>> a = [5,2,8,3,1]
     >>> sorted(a)
     [1, 2, 3, 5, 8]
     >>> sorted(a,reverse=True)
     [8, 5, 3, 2, 1]
    
  4. Reversing a List
     >>> a = [5,2,8,3,1]
     >>> a[::-1]
     [1, 3, 8, 2, 5]
    
  5. Split data and assign values to variables
     >>> (age, income) = "35,150000".split(',')
     >>> age
     35
    
  6. Access Key and Value from a list using for loop
     mydict = {
         "a":"cat",
         "b":"dog"
     }
    	
     for key,val in mydict.items():
         print("key:", key, "value:", val)
    
  7. Access items of two lists of same sizes at the same time
     a = [4,5,6]
     b = [1,2,3]
     for i,j in zip(a,b):
         print(i+j)
     # output
     # 5 7 9
    
  8. Create a string from the elements in list and Vice-versa
     mylist = ["hi", "there", "!"]
     string1 = " ".join(mylist)
     string2 = "".join(mylist)
     print(string1)
     print(string2)
     # output
     # hi there !
     # hithere!
    
     newlist = string1.split(" ")
     print(newlist)
     # output
     # ["hi", "there", "!"]
    
  9. List comprehension makes the code shorter
    myint = [2,4,6]
    mysquare = []
    for i in myint:
        mysquare.append(i*i)
    print(mysquare)
    # output: [4, 16, 36]
    

    same thing can be done in one line

    myint = [2,4,6]
    mysquare = [i*i for i in myint]
    print(mysquare)
    # output: [4, 16, 36]
    
  10. Get arguments from the terminal and put in a list
    import sys
    args = [int(arg) for arg in sys.argv[1:]]
    print(sum(args))
    

    now, save the code in a file (here, test.py) and run like this

    $ python test.py 2 4 6 8
    20
    
  11. Single line function using Python Lambda
    # an example function using def
    def testfunc(a,b):
        return a**2 + b**2, a**3 + b**3
    
    x,y = testfunc(2,3)
    print(x,y)
    # output: 13 35
    

    now, let’s create the above function using lambda

    testfunc = lambda a,b: (a**2 + b**2, a**3 + b**3)
    x,y = testfunc(2,3)
    print(x,y)
    # output: 13 35
    
  12. map, filter, reduce
    a = [1,2,3,4,5]
    
    square = map(lambda x:x**2, a)
    print(list(square))
    
    even = filter(lambda x:x%2==0, a)
    print(list(even))
    
    from functools import reduce
    summ = reduce(lambda x,y:x+y,a)
    print(summ)
    	
    # output
    # [1, 4, 9, 16, 25]
    # [2, 4]
    # 15 
    
  13. Create a dictionary from two lists using dictionary comprehension
    a = [1,2,3,4,5]
    b = ["a", "b", "c", "d", "e"]
    mydict = {i:j for i, j in zip(a,b)}
    print(mydict)
    # output: {1: 'a', 2: 'b', 3: 'c', 4: 'd', 5: 'e'}
    
  14. Sort a Dictionary by value
    sorted_dict = {k:v for k,v in sorted(my_dict.items(), key=lambda item: item[1])}
    
  15. List edges of a graph using nested comprehension
    G = {
        "a": ["b","c"],
        "b": ["d", "e"],
        "c": ["d", "e"],
        "d": ["e"],
        "e": ["f"]
    }
    
    def edgelist(graph):
        edges = [(key,i) for key,val in graph.items() for i in val]
        return edges
    
    print(edgelist(G))
    # output: [('a', 'b'), ('a', 'c'), ('b', 'd'), ('b', 'e'), ('c', 'd'), ('c', 'e'), ('d', 'e'), ('e', 'f')]
    
  16. Flattening a list using itertools
    from itertools import chain
    print(list(chain.from_iterable([[1, 2], [3], [4], [5, 6]])))
    # output: [1, 2, 3, 4, 5, 6]
    
  17. use of arguments and keyword arguments in functions
    from functools import reduce
    def mysum(first, second, *args, kw1, **kwargs):
        summ = first + second + reduce(lambda x,y:x+y, [i for i in args]) + kw1 + kwargs["end"]
        return summ
    print(mysum(3, 4, 5, 6, 7, kw1=8, end=10))
    
  18. returning function from a function
    def myfunc():
        def new():
            print('hi there!')
        return new
    
    a = myfunc()
    a()
    # output: hi there!
    
  19. Use of continue and break in loops
    for i in range(10):
        if (i is 1):
            continue
        if (i > 5):
            break
        print(i)
    # output: 0 2 3 4 5
    
  20. Calculate runtime of a function using decorator
    from functools import wraps
    from time import time
    
    def runtime(func):
        @wraps(func)
        def wrapper(*args, **kw):
            start = time()
            result = func(*args, **kw)
            end = time()
            print('runtime: %2.4f sec' %(end-start))
            return result
        return wrapper
    
    @runtime
    def mysum(myrange):
        summ = 0
        for i in range(myrange+1):
            summ += i
        print("sum =", summ)
    
    mysum(5)
    # sum = 15
    # runtime: 0.0002 sec
    

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