15-437 Python

Transcript

1. Python Refresher Thierry Sans
2. Hello World
3. What Java is like class HelloWorld { static public void main( String args[] ) { System.out.println( "Hello World!" ); } }
4. What Python is like print "Hello World"
5. Python Programs
6. Interacting with Python • Interactive shell • Files • Executable files $ python >>> print "Hello World" $ python helloWorld.py $ chmod +x helloWorld.py $ ./helloWorld.py #! /usr/bin/env python print "Hello World" helloWorld.py print "Hello World" helloWorld.py
7. Basic (Immutable) Types
8. Integers and Floats • Integers • Floats 2 + 3 # returns 5 7 / 3 # returns 2 7 % 3 # returns 1 3. / 2. # returns 0.5 3. / 2 # returns 0.5 float(1) / 2 # returns 5
9. Booleans • Values and operators • Equality and inequality True # is actually 1 False # is actually 0 a and b a or b not a a == b a != b
10. Strings “hello” + “world” # returns “helloworld” “hello” * 3 # returns “hellohellohello” “hello”[0] # returns “h” “hello”[1:4] # returns “ell” “hello”[-1] # returns “o”, last element len(”hello”) # returns 5 ”sml” < ”python” # returns 0 (False) ”e” in ”hello” # returns 1 (True) ”\”hello\”” # returns “hello”
11. Tuples and named tuples () # returns () (“hello”, 3) # returns (“hello”,3) (“hello, 3)[0] # returns “hello”
12. (Mutable) Data Structures
13. Lists a = [“I”, “have”, 3, “QAR”, [“in”,”my”,”pocket”]] a + [“!”] # concatenation a[0] # indexing a[1:4] # slicing len(a) # length del(a[-1]) # delete range(3) # returns [0,1,2] range(3:5) # returns [3,4]
14. Object-style methods a.append(5) # inserts 5 at the end a.insert(0,5) # inserts 5 at index 0 (shift) a.pop() # returns the last element # and deletes it a.pop(0) # returns the element at index i # and deletes it (shift) a.reverse() # reverse a.sort() # sort
15. Dictionaries (aka associative arrays or hashtables) a = {“name”:“John”, “age”: 5} a[“name”] # returns “john” (lookup) a[“address”] # returns KeyError exception del(a[“name”]) # delete a[“city”]=”doha” # insert a[“city”]=”paris” # modify • Values can be anything (mix and match) • But keys must be immutable and of the same type
16. Object-style methods a.keys() # [“age”,”city”] a.values # [5,”paris”] a.items() # [(“age”,5),(“city”,“paris”)] a.has_key(“name”) # returns false
17. Types, Variables and References
18. Variable • A variable is declared when first assigned to a value • Unknown variables raise exceptions
19. Types • No explicit type declaration • But implicit typing • Wrong typed expressions raise exceptions
20. References • Modifying an immutable variable • A new reference is created • Modifying a mutable variable • The value pointed by the reference is changed
21. Flow Controls
22. If statement if condition: statements [elif condition: # optional statements] [else: statements] # optional
23. While statement while condition: statements
24. For and range for k in range(10) print k
25. Functions
26. Functions def name(arg1,arg2=default): “documentation” statements
27. Example def gcd(a,b): “greatest common divisor” while a!=0: a,b = b % a, a return b gcd(12,20) # returns 4 gcd.__doc__ # returns “greatest common divisor”
28. Function vs Procedure def name(arg1,arg2): statements return # defines a procedure def name(arg1,arg2): statements return something # defines a function
29. Lambda form var name = lambda x,y: x+y
30. Exceptions
31. Raising exception raise MyException('Houston, we’ve got a problem!')
32. Catching exception def divisionBy0(x) try: print 1 / x except ZeroDivisionError, message: print “Can’t divide by zero:”, message else: print “Can divide by zero” finally: print “done”
33. Object Oriented Programming
34. Defining a class class Stack: "A well-known data structure..." def __init__(self): self.items = [] # constructor def push(self, x): self.items.append(x) def pop(self): x = self.items[-1] # what happens if it’s empty? del self.items[-1] return x def empty(self): return len(self.items) == 0 # Boolean result
35. Instantiating an object x = Stack() # new object of type Stack x.empty() # returns True x.push(1) x.empty() # returns false x.push("hello") x.pop() # returns “hello”
36. Modules
37. Module made easy • A module is a collection of classes, functions and variables • Creating a module • Create a file name stack.py (containing the stack definition defined previously) • Using a module import stack # import all definitions in stack x = stack.Stack() from stack import Stack # import the definition of Stack only x = stack.Stack() Or
38. Executing a module # Fibonacci numbers module def fib(n): # write Fibonacci series up to n a, b = 0, 1 while b < n: print b, a, b = b, a+b if __name__ == "__main__": import sys fib(int(sys.argv[1])) fibo.py Execute the module $ python fibo.py 4
39. Pros and Cons
40. Python or not Python? ✓ Syntax friendly ✓ Good libraries ๏ Dynamically typed (hard to debug)