# PythonIntro.py (Source)

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This is a sample code to illustrate some basic features of the Python language. This notebook was created as supplemental material to a Python for financial research bootcamp for finance honours and PhD students at the University of Melbourne in March of 2017. Last update: March 24, 2017. **Contact**: Latest version: """ # This is a comment! """ This is a longer comment that can span multiple lines. Actually this is a string which is not assigned to anything, so it is code, but it does nothing. """ # %% # It is usual to always begin by saying Hello World! print "Hello World!" # %% This is a neat comment trick that split blocks of code in Spyder # To execute the code in the block, just type shift-enter. # I prefer to use parenthesis, but they are not required for print. print('Hello World!') # Python is case sensitive, so the statement below is different from # the one above, and would not work as PRINT() is not defined. # PRINT('HELLO WORLD!') # %% Number and operators print(2 + 2) print(4 - 1) print(5 * 2) print(5.5 * 2) print(11.0 / 2) print(11 / 2) # Python 3 converts automatically to floating point if required # Not no in Python 2, but we can activate this feature for compatibility. from __future__ import division print(11 / 2) print(2**8) print(round(22.34)) print(8//3) # Integer division print(8%3) # Modulo (remainder of integer division) # %% Variables a = 2 b = 4 a * b c = 'Hello' c # This doesn't work # a + c d = "World" c + ' ' + d + '!' # %% Operators 2 == 2 2 < 2 2 <= 2 True | False # OR True & False # AND True != False # XOR not True # NOT # %% None # In Python there is a special value for null objects that is called None. # Note that you first need to define a variable as None before you can # access it. a = None b = 1 a is None b is None b is not None # %% Strings s = 'Hello World!' len(s) s[1] s[0] s[-1] s[-2] s[2:5] s.upper() s.lower() ' Hello '.strip() '2'.zfill(3) '20'.zfill(3) s.replace('World', 'Moon') s.replace('o', 'a') s.find('d') s[10] # s + 2 # Error s + str(2) s * 2 s.split(' ') s.startswith('Hell') s.count('l') print('Don\'t forget to escape the quotes.') #%% Input x = raw_input('Type something: ') #%% Input exercise # Exercise: write a program that asks the user's name and responds with # a personalised greating. x = raw_input('What is you name? ') print('Good day to you, ' + x + '.') # %% Sequences # List l = [2, 4, 'blabla', 4] l[0] l[2:4] l[0] = 3 # Delete one element del l[1] l.append(3) l.reverse() l.sort() l.pop() l + l # Tuple (immutable list) t1 = 2, 3 t2 = (2, 3) 34 not in t1 3 in t1 # Set (unordered list of unique elements) {3, 4, 4, 5, 4} # Dictionnary d = {1: 2, 'a': 'Hello'} d['a'] d['b'] = 'World!' # Special note: copying lists (and most other objects) # This does not create a new list, both l and l2 refer to the # same list. l2 = l l2.reverse() l # To create a new copy, you need to deepcopy import copy l3 = copy.deepcopy(l) # %% Identity # For base types (numbers, strings), 'is' and == behave the same way. 2 is 2 2 == 2 'aa' is 'aa' 'aa' == 'aa' # For other objects (including list), 'is' checks if the two operands refer # to the same object, while == compares the content. [1, 2, 3] is [1, 2, 3] [1, 2, 3] == [1, 2, 3] # %% Loops and branches (Flow control) # Notice the indentation! This is how Python identifies # blocks of codes. PEP8 recommends 4 spaces (as long as a tab, # but actual spaces.) Spyder will do this automatically. # Branches if 1 > 2: print('Yes') elif 2 > 2: print('No') else: print('I don\'t know') # Repeat for each element in the list. (note that 'l' need not be a list, it # can be anything that is iterable, but for now you can think list!) for x in l: print(x) range(5) for i in range(5): print(i) for i, x in enumerate(l): print( str(i) + ', ' + str(x) ) # Interating on dictionnaries is iterating on keys. for x in d: print(str(x) + ': ' + str(d[x])) # Repeat something until a condition is met. sum = 0 while sum < 10: sum = sum + 2 print(sum) # break and continue # When iterating, you can stop the loop completely using break, # or just skip the current iteration using continue. for i in range(10): if i == 3: continue elif i == 6: break print(i) # %% List comprehension # We saw above how to assign a list statically. Another way is to assign # actively from another list fillings = ['Apple', 'Cherry', 'Sugar', 'Meat'] pies = [x + ' pie' for x in fillings] # Same idea with dictionnaries swapped_cases = {x: x.swapcase() for x in fillings} # Can have conditions desserts = [x + ' pie' for x in fillings if x != 'Meat'] # Or multiple loops desserts = [x + ' pie' + y for x in fillings if x != 'Meat' for y in ('', ' with ice cream')] # You can have as many loops as you want, but best practice is to limit to # only 2 loops in list comprehensions. # %% Loading packages # Python is a pretty powerful language, but most of the benefits come from # the buil-in functionality is somewhat limited. Most advanced features are # available through packages, either as part of the Python Standard Library # or from third-parties. # To load a package and make it available, you have to import it. import sys # You can list the components using the dir() method (or in the console, # typing os. and then tab to show avaialble options.) dir(sys) sys.path sys.getrecursionlimit() # It's common to rename packages with the 'as' keyword to make access easier. import pandas as pd # If you want to import all the components of a pakcage, you can use * from datetime import * # But it's usually best to only import what you need from datetime import datetime, timedelta # You can also load only a subpackage if that's all you need. import statsmodels.formula.api as sm # %% Functions def square(x): return x * x square(2) # Variables defined inside a function are only available # inside that function def computeBigSum(a, b, c, d): e = a + b f = c + d print(str(e)) # This works here return e, f sum1, sum2 = computeBigSum(1, 2, 4, 5) # But not here. # print(str(e)) # You have to use a name defined in this scope. print(str(sum1)) # You can make some arguments optional by assigning default values. # Note that all required arguments need to be listed before the # optional ones def raise_power(n, power=2): return n**power print(raise_power(4)) print(raise_power(4, 2)) print(raise_power(4, 3)) # %% Recursive function # A function can call itself, we call this a recursive # function. Just make sure you have a condition to stop # the recursion (so that it doesn't run forever). # A Fibonacci number is part of a sequence were every number is the # sum of the previous two numbers in the sequence. The first two numbers # of the sequence are defined to be 0 and 1. # https://en.wikipedia.org/wiki/Fibonacci_number def fibonacci_recursive(n): if n in [0, 1]: return n return fibonacci_recursive(n - 1) + fibonacci_recursive(n - 2) fibonacci_recursive(10) # %% Iterative function # However, recursion is not always the best solution. In this case, for # large numbers (say 50+), even a modern computer will take some time. # Exercise: fund and code the better way! (Google search allowed!) # A better way is the iterative way def fibonacci_iter(n): if n in [0, 1]: return n a = 0 b = 1 c = a + b for i in range(2, n): a = b b = c c = a + b return c fibonacci_iter(10) [fibonacci_iter(x) for x in range(10)] # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34] # %% Exceptions # Sometimes things go wrong, and the code crashes. But it doesn't have to. # You can catch exceptions, and decide what to do. def inverse(n): return 1.0 / n inverse(0) def inverse_ex(n): out = None try: out = 1.0 / n except ZeroDivisionError as e: print(e[0]) out = 0.0 finally: return out inverse_ex(0) # Anywhere in you code, you can raise your own exceptions raise Exception('I just raised an exception!') # %% Classes # Python is an object-oriented programming # language, which means that most of the datatypes are classes, and the # actual instances of those datatypes are called objects. # You can create your classes if you want to (probably won't need to at first) class Rectangle: def __init__(self, length=1, width=1): self.length = length self.width = width def __repr__(self): return str(self.length) + 'x' + str(self.width) + ' rectangle' def getArea(self): return self.length * self.width def setLength(self, x): self.length = x def setWidth(self, x): self.width = x def getLength(self): return self.length def getWidth(self): return self.width rect = Rectangle(3, 4) print(rect) rect.getArea()