CS2304: Python for Java Programmers. CS2304: Advanced Function Topics

Size: px

Start display at page:

Download "CS2304: Python for Java Programmers. CS2304: Advanced Function Topics"

Dora Mathews
6 years ago
Views:

1 CS2304: Advanced Function Topics

2 Functions With An Arbitrary Number of Parameters Let s say you wanted to create a function where you don t know the exact number of parameters. Python gives you a few ways to this, here s the first way: def example(*args): for arg in args: print(arg, end= ) # in the interactive interpreter >>> example(1) 1 >>> example(1, hello ) 1hello

3 Functions With An Arbitrary Number of Parameters Having a * in front args tells Python to take all of the given parameters and put them in a list. Think back to tuples, you could unpack them using a * as well. def example(*args): for arg in args: print(arg, end= ) # in the interactive interpreter >>> example(1) 1 >>> example(1, hello ) 1hello

4 Related: Arbitrary Parameters Names (and Numbers) Using ** on a parameter gives you the keyword arguments and values inside of a dictionary. So a function can actually also accept arbitrary parameter names and values. def example2(**kwargs): for k,v in kwargs.items(): print(k + : + str(v)) # in the interactive interpreter >>> example2(name= John, id=0, date= 3/31/14 ) name:john date:3/31/14 id:0

5 Putting Them All Together We can actually combine standard parameters with *args and **kwargs: def example3(param1, *args, **kwargs): print( Param1 is + str(param1)) for arg in args: print(arg) for k,v in kwargs.items(): print(k + : + str(v))

6 Putting Them All Together We can actually combine standard parameters with *args and **kwargs: # in the interactive interpreter >>> example2(1, h, name= John, id=0, date= 3/31/14 ) Param1 is 1 h name:john date:3/31/14 id:0

7 First-class Functions In Python, functions are considered first-class citizens. What does that mean? Here s a description from Wikipedia: this means the language supports passing functions as arguments to other functions, returning them as the values from other functions, and assigning them to variables or storing them in data structures So a function is pretty much like every other variable we ve encountered.

8 Function Are Really Objects Just like strings, lists and tuples. So all of this works fine: >>> def func(): print("hello")... >>> def takes_func(f): return f... >>> f = takes_func(func) >>> f <function func at 0x1006bf560> >>> f() Hello >>> func() Hello

9 Passing A Function As A Parameter You might have a function as a parameter to enable generic programming. As an example, one parameter might be a function to process custom data types. Or let the user modify the result of your function. So this is a (slightly) less contrived example: def handle_user_input(data, process=none): if process == None: # handle data the standard way else: process(data)

10 Lambda Expressions Lambda expressions are anonymous functions. They can be used where ever you need a function object. They start with the lambda keyword, then have: a list of parameters, a colon, then a single statement. f = lambda a, b: a + b >>> f(1, 2) 3

11 Hint: A Project Related Example Let s say you had a list of dictionaries you needed to sort, like so: l = [{"year":1995, "name":"john"}, {"year":1996, "name":"bob"}, {"year":1997, "name":"hello"}] sorted is a built-in sorting function, but we need to tell it how to sort the dictionaries: >>> sorted(l, key=lambda data: data["year"]) [{'name': 'John', 'year': 1995}, {'name': 'Bob', 'year': 1996}, {'name': 'Hello', 'year': 1997}] >>> sorted(l, key=lambda data: data["name"]) [{'name': 'Bob', 'year': 1996}, {'name': 'Hello', 'year': 1997}, {'name': 'John', 'year': 1995}]

12 Making An Object Callable By adding a call method we can treat any object like a function: class hello: def init (self): self.j = 0 def call (self): self.j += 1 return self.j Now I can do this: >>> h = hello() >>> h() 1 >>> h() 2

13 Returning A Function You can use Lambba functions to create then return a function: def generate_function(i) return lambda x: x + i >>> f = generate_function(100) >>> f(10) 110 >>> f(20) 120

14 Remapping A Function Since functions are objects like everything else you can remap a name to an another function like standard variables. callme = None: if input == even : callme = even_func else: callme = odd_func callme() callme = lamba a: str(a)

LECTURE 2. Python Basics

LECTURE 2. Python Basics LECTURE 2 Python Basics MODULES ''' Module fib.py ''' from future import print_function def even_fib(n): total = 0 f1, f2 = 1, 2 while f1 < n: if f1 % 2 == 0: total = total + f1 f1, f2 = f2, f1 + f2 return