Lessons on Python Functions

Transcription

1 Lessons on Python Functions Walter Didimo [ 90 minutes ]

2 Functions When you write a program, you may need to recall a certain block of instructions several times throughout your code A function is a block of instructions grouped together in a unique name you can use this name to recall such a block

3 How to define a function General syntax of a function name of the function input taken by the function def <func_name>(<parameters>): <docstring> <instructions> string that describes the function body of the function

4 Example Function that prints all even integers in a given interval def printevennumbersbetween(a,b): """Print all even integer numbers in the specified interval [a,b]: PRE: b > a""" if a%2 == 0: start = a else: start = a+1 for i in range(start,b+1,2): print(i)

5 Docstring The description used in the <docstring> part is a comment between triple quotes, called the docstring of the function It is a property of the function, which can be accessed with the following syntax <func_name>. doc >>> print(printevennumbersbetween. doc ) Print all even integer numbers in the specified interval [a,b]: PRE: b >= a

6 Variables in a function Variables that receive a value within a function, are treated as local variables a = 10 def f1 (): a = 5 f1() print(a) 10 Calling f1 does not change the value of a at the top level; the variable a inside f1 is a different one

7 Variables in a function: global However, the global statement can be used within a function to refer to an external global variable a = 10 def f1 (): f1() global a a = 5 print(a) 5 This time the value of the global variable a will be modified by f1

8 Variables in a function: global You can define a global variable inside a function even if such a function is not explicitly defined outside; in this case the same variable can be used in other functions of the same module (see later), still using the prefix global def f1 (): global a a = 5 def f2 (): global a print(a) f1(); f2() 5

9 The return statement The return statement can be used to interrupt a function and to return a value def intnumbers(n): """Return all integers in [0,n]""" nums = [] for i in range(n+1): nums.append(i) return nums >>> print(intnumbers(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

10 The None value If a function does not contain a return statement, it implicitly returns the built-in None value def printintnumbers(n): """Print all integers in [0,n]""" for i in range(n+1): print(i,end="-") >>> print(printintnumbers(10)) None

11 Function parameters The parameters of a function are used to pass values to it when the function is called The actual parameters passed to a function are called arguments All arguments are passed by value each argument is just copied into its corresponding parameter When you pass an object to a function, you are in fact passing a reference to that object not a copy of the object

12 Function parameters example def mergelists(l1,l2): """Return a list that merges l1 and l2""" merged = [] for i in l1: merged.append(i) for i in l2: merged.append(i) return merged >>> l1, l2 = [1,2,3], [4,5,6] >>> print(mergelists(l1,l2)) [1,2,3,4,5,6]

13 Function parameters example def merge(l1,l2): """Merge l2 with l1: l1 is modified""" for i in l2: l1.append(i) return l1 >>> l1, l2 = [1,2,3], [4,5,6] >>> print(merge(l1,l2)) [1,2,3,4,5,6] >>> print(merge(l1,l2)) [1,2,3,4,5,6,4,5,6]

14 Checking arguments In Python you can only specify the name of a function parameter, but not its type This may lead to an execution error when your function expects a specific type of parameter but receives a different one In the function's body you can check the type of an argument using the type() function you can even write a function that accepts different types of arguments for a certain parameter and that differentiates its behavior depending on the parameter type

15 Checking arguments - example The following method joins (i.e., concatenate) strings and/or numbers def join(e1,e2): if (type(e1) == type(0)): #e1 is a number e1 = str(e1) if (type(e2) == type(0)): #e2 is a number e2 = str(e2) if (type(e1)!=type("") or type(e2)!=type("")): print("invalid parameters") e = None else: e = e1 + e2 return e

16 Variable number of arguments You can also define functions that can be called with a variable number of arguments We explain the following concepts: default arguments keyword arguments arbitrary argument sets

17 Default arguments When you define a function, you can assign a default argument to some parameters: a default argument is used if and only if no other value is specified for its parameter when the function is called def decimaldigits(num,d=2): """Return a string representation of num, rounded to d decimal digits""" decdig = "%."+str(d)+"f" rounded = decdig % num return rounded

18 Default arguments example 1 def decdigits(num,d=2): """Return a string representation of num, rounded to d decimal digits""" decdig = "%."+str(d)+"f" rounded = decdig % num return rounded >>> decdigits( ) >>> decdigits( ,3)

19 Default arguments example 2 def decdigitsandnotation(num,d=2,n='f'): """Return a string representation of num, rounded to d decimal digits and using the n notation""" decdig = "%."+str(d)+n rounded = decdig % num return rounded >>> decdigitsandnotation( ) >>> decdigitsandnotation( ,2,'e') '3.06E+01'

20 Default arguments warning! Warning: a default argument is evaluated only once! This may lead to unexpected behaviors when the default argument is a mutable object (e.g., a list or a dictionary, or some other class instance) def addtolist(num,lis=[]): """append num to lis""" lis.append(num) return lis >>> addtolist(1); addtolist(2); addtolist(3) [1] [1, 2] [1, 2, 3]

21 Keyword arguments So far we have specified the arguments of a function only according to a positional criterion It is also possible to specify an argument through a keyword, i.e., using the symbolic name of its parameter Keyword arguments can be specified in any order, but all of them must occur after the positional arguments

22 Keyword arguments example def decdigitsandnotation(num,d=2,n='f'): """Return a string representation of num, rounded to d decimal digits and using the n notation""" decdig = "%."+str(d)+n rounded = decdig % num return rounded >>> decdigitsandnotation( , n='e') >>> decdigitsandnotation( , n='e', d='3') >>> decdigitsandnotation(n='e', num= , d='3') >>> decdigitsandnotation(n='e', ) invalid call valid calls

23 Arbitrary argument sets Python also supports parameters that can have an arbitrary number of arguments *args: accepts an arbitrary number of arguments, which will be wrapped into a tuple **keyargs: accepts an arbitrary number of keyword arguments, which will be wrapped into a dictionary A **keyargs parameter must follow a *args parameter, and both of them must follow any other type of parameter

24 Arbitrary argument sets - example def createdict(mess, *listelem, **dicelem): print(mess) for m in listelem: print(m) for i in dicelem: print(i,'-->',dicelem[i]) return dicelem >>> print(createdict("my dictionary", "Syntax:", "keyword --> value", one='1', two='2', three='3'))

25 Arbitrary argument sets - example >>> print(createdict("my dictionary", "Syntax:", "keyword --> value", one='1', two='2', three='3')) My dictionary Syntax: Keyword --> value one --> 1 two --> 2 three --> 3 {'one': '1', 'two': '2', 'three': '3'}

26 Unpacking argument lists When you invoke a function and you need to pass to it all the elements of a tuple or of a list in an unpacked form, use the * operator >>> print(*[10,20,30])

27 Copying function names It is possible to copy the name of a function into another name: in this case, both the two names can be used to invoke the function For instance, the built-in functions min() returns the minimum value of an iterable object; the next code defines an alternative name for this function. >>> minimum = min >>> minimum([10,20,-1,-2,30,0]) -2

28 Lambda expressions A lambda expression can be used to define a small anonymous (without name) function list of parameters returned value lambda <parameters>: expression The parameters are separated by commas, as usual expression must be a simple expression, without other types of instructions

29 Lambda expressions - examples >>> g = lambda x,y,z: x+y+z >>> g(10,20,30) 60 >>> f = lambda m: 2*m >>> f(g(10,20,30)) 120

30 Lambda expressions - examples Anonymous functions can also be used as a return value of another function (i.e., a function returns another function) >>> def f(n): return lambda m: n+m >>> g = f(10) >>> g(20) 30 or equivalently >>> f(10)(20) 30

31 Lambda expressions - examples A lambda expression can also be the attribute of another function; the following functions allows you to execute any function f on a parameter n >>> def myfunc(f,n): return f(n) >>> myfunc(lambda x:x**2,100) >>> 10000

32 Lambda expressions - examples Another example >>> lis = [(1,2,3), (2,1,4), (3,3,0)] >>> lis.sort(key=lambda triple: triple[2]) >>> lis [(3, 3, 0), (1, 2, 3), (2, 1, 4)] We now see two useful built-in functions, which can take as input lambda functions

33 The filter() function filter(function,iterable) returns a new iterable object (filter) that represents the subset of the elements in the iterable object for which function returns True For instance, the following code filters the elements that correspond to even numbers >>> L = [10,25,4,6,57] >>> even = list(filter(lambda x: x%2==0,l)) >>> even [10,4,6]

34 The map() function map(function,iterable) returns a new iterable object (map) obtained by applying function to all elements of iterable For instance, the following code can be used to get a list of squared elements from an initial list >>> L = [1,2,3,4,5] >>> squares = list(map(lambda x: x**2,l)) >>> squares [1, 4, 9, 16, 25]

35 The map() function This is another example, which shows you the power of this language: L is a list of functions the parameter of the lambda function in the map function is a function itself! >>> double = lambda x: x*x >>> triple = lambda x: pow(x,3) >>> L = [double, triple] >>> for n in range(1,4): >>> print(list(map(lambda f: f(n),l))) [1, 1] [4, 8] [9, 27]

36 Recursive functions Recursion is an important technique that reflects mathematical induction A recursive function is a function that calls itself Example: The following recursive function computes the sum of the first n integer numbers, where n is a nonnegative integer 0 if n = 0 base case sum(n) = n + sum(n-1) if n > 0 inductive case sum(3) = 3 + sum(2) = sum(1) = sum(0) = = 6

37 Recursive functions - example This is an implementation of the sum() recursive function def sum(n): """ Recursively compute the sum of the integer numbers in the interval [0,n]""" s = -1 #initially not defined if n==0: s = 0 elif n>0: s = n + sum(n-1) else: # n is negative return s print("error: n cannot be negative!")