Python Puzzlers - 2016 Edition

Python PuzzlersFeb 17, 2016

Nandan Sawant, Ryan Rueth

1. Defaulter

Defaulter

def extend_list(val, l=[]):

l.append(val)

return l

list1 = extend_list(10)

list2 = extend_list(123,[])

list3 = extend_list('a')

print "list1 = %s" % list1



Defaulter

def extend_list(val, l=[]):

l.append(val)

return l







list1 = [10, 'a']

list2 = [123]

list3 = [10, 'a']

Defaulter

From the python documentation:https://docs.python.org/2/reference/compound_stmts.html#function-definitions

● Avoid mutable default arguments● If you use mutable default arguments,

○ don’t mutate them○ have a good reason to mutate them (e.g. caching)

https://docs.python.org/2/reference/compound_stmts.html#function-definitions

https://docs.python.org/2/reference/compound_stmts.html#function-definitions

Defaulter

def extend_list(val, l=None):

if l is None:

l = []

l.append(val)

return l







list1 = [10]

list2 = [123]

list3 = ['a']

2. Who’s late?

Who’s late?

multipliers = [lambda x: (i+1)*x for i in range(10)]

def print_multiplication_table_for(number):

print [multiplier(number) for multiplier in multipliers]

print_multiplication_table_for(2)

Who’s late?

multipliers = [lambda x: (i+1)*x for i in range(10)]




[20, 20, 20, 20, 20, 20, 20, 20, 20, 20]

Who’s late?

multipliers = []

for i in range(10):

def multiplier(x):

return (i+1) * x

multipliers.append(multiplier)




Who’s late?

multipliers = []

for i in range(10):

def multiplier(x):

return (i+1) * x

multipliers.append(multiplier)




[20, 20, 20, 20, 20, 20, 20, 20, 20, 20]

● A closure occurs when a function has access to a local variable from an enclosing scope that has finished its execution

● Closures in python are late-binding

● This means that the values of variables used in closures are looked up at the time the closure is called

● This behavior is NOT specific to lambdas

Who’s late?

Who’s late?

multipliers = [lambda x, i=i: (i+1)*x for i in range(10)]




[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

Who’s late?

from functools import partial

from operator import mul

multipliers = [partial(mul, i+1) for i in range(10)]




[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

3. Western Addition

Western Addition (A)


a = b = [1,2,3]

a += [4]

print a, b

a = a + [5]

print a, b


a = b = [1,2,3]

a += [4]

print a, b

a = a + [5]

print a, b

[1, 2, 3, 4] [1, 2, 3, 4]

[1, 2, 3, 4, 5] [1, 2, 3, 4]

● a + b uses __add__ while a += b uses __iadd__

● An object's __add__ method is regular addition: it takes two parameters, returns their sum, and doesn't modify either parameter

● An object's __iadd__ method also takes two parameters, but makes the change in-place, modifying the contents of the first parameter


Western Addition (B)


a = b = 'western'

a += 'addition'

print a, b

a = a + 'sf'

print a, b


a = b = 'western'

a += 'addition'

print a, b

a = a + 'sf'

print a, b

westernaddition western

westernadditionsf western

● a + b uses __add__ while a += b uses __iadd__, if it exists

● An object's __add__ method is regular addition: it takes two parameters, returns their sum, and doesn't modify either parameter

● An object's __iadd__ method also takes two parameters, but makes the change in-place, modifying the contents of the first parameter

● Because this requires object mutation, immutable types (like Strings!) don’t have an __iadd__ method


Western Addition (C)


a = ([1,2,3],)

a[0] += [4]

print a


a = ([1,2,3],)

a[0] += [4]

print a

TypeError: 'tuple' object does

not support item assignment

([1, 2, 3, 4],)

● An object's __iadd__ method takes two parameters, but makes the change in-place, modifying the contents of the first parameter and returns the first parameter

● The code above is equivalent to:


a = ([1,2,3],)

x = a[0]

x = x.__iadd__([4]) # returns x itself

a[0] = x

Western Addition (D)


a = b = 500

print a is b

a -= 200

b -= 200

print a is b

a = a - 200

b = b - 200

print a is b

--a

--b

print a is b


a = b = 500

print a is b

a -= 200

b -= 200

print a is b

a = a - 200

b = b - 200

print a is b

--a

--b

print a is b

True

False

True

True

From the python documentation:https://docs.python.org/2/c-api/int.html


● There is no decrement (--) operator in python● -- is simply two negation operators

https://docs.python.org/2/c-api/int.html



4. Crossing the Boundary

Crossing the Boundary (A)


a, b = 0, 1

def fib(n):

for i in range(n):

a, b = b, a + b

return a

print fib(4)


a, b = 0, 1

def fib(n):

for i in range(n):

a, b = b, a + b

return a

print fib(4)

UnboundLocalError: local

variable 'b' referenced before

assignment


x = 1

def f1():

print x

f1()

print x

x = 1

def f2():

x = 2

print x

f2()

print x

x = 1

def f3():

print x

x = 3

f3()

print x


x = 1

def f1():

print x

f1()

print x

x = 1

def f2():

x = 2

print x

f2()

print x

x = 1

def f3():

print x

x = 3

f3()

print x

1

1

2

1

UnboundLocalError: local

variable 'x' referenced

before assignment

1

● If we want to access a global variable from inside a function, it is possible

● The assignment statement inside a function creates variables in the local scope

● If a local variable has the same name as a global variable the local variable will always take precedence

● The assignment inside the function does not modify the global variable

● We may not refer to both a global variable and a local variable by the same name inside the same function, it gives us an error

● Deep Dive http://eli.thegreenplace.net/2011/05/15/understanding-unboundlocalerror-in-python


http://eli.thegreenplace.net/2011/05/15/understanding-unboundlocalerror-in-python


a, b = 0, 1

def fib(n):

for i in range(n):

global a, b

a, b = b, a + b

return a

print fib(4) 3

Crossing the Boundary (B)

Crossing the Boundary (C)


main.py

try:

import quitter

except:

exit(0)

quitter.quit()

quitter.py

def quit():

exit(0)

# Add one indented line here

# to make main.py crash

python main.py


main.py

try:

import quitter

except:

exit(0)

quitter.quit()

quitter.py

def quit():

exit(0)

exit = None

python main.py


main.py

try:

import quitter

except:

exit(0)

quitter.quit()

quitter.py

def quit():

exit(0)

exit = None

UnboundLocalError: local variable

'exit' referenced before assignment

python main.py

5. Kids These Days

Kids These Days (A)

Kids These Days (A)

class Parent(object):

x = 1

class Child1(Parent):

pass


pass

print Parent.x, Child1.x, Child2.x

Child1.x = 2


Parent.x = 3


Kids These Days (A)


x = 1


pass


pass


Child1.x = 2


Parent.x = 3


1 1 1

1 2 1

3 2 3

● In Python, class variables are internally handled as dictionaries.

● If a variable name is not found in the dictionary of the current class, the class hierarchy (i.e., its parent classes) are searched until the referenced variable name is found

● If the referenced variable name is not found in the class itself or anywhere in its hierarchy, an AttributeError occurs

Kids These Days (A)

Kids These Days (B)

Kids These Days (B)

class Parent:

x = 1


pass


x = 2

class GrandChild(Child1, Child2):

pass

print GrandChild.x

Kids These Days (B)

class Parent:

x = 1


pass


x = 2


pass

print GrandChild.x 1

Kids These Days (B)


x = 1


pass


x = 2


pass

print GrandChild.x 2

● If you don’t inherit from ‘object’, you’re defining an old style class

● MRO is Method Resolution Order. Lookup __mro__

● MRO for old style classes follows a native DFS approachGrandchild -> Child 1 -> Parent -> Child 2 -> Parent

● MRO for new style classes is more sensibleGrandchild -> Child 1 -> Child 2 -> Parent

● Don’t use old-style classes

Kids These Days (B)

6. Exceptional Circumstances

Exceptional Circumstances

user_id = 'a'

try:

user_id = int(user_id)

print user_id

except TypeError, ValueError:

print "Oops!"

(a) a(b) 65(c) Oops!(d) raises TypeError(e) raises ValueError


user_id = 'a'

try:


print user_id

except TypeError, ValueError:

print "Oops!"


ValueError: invalid literal

for int() with base 10: 'a'


user_id = 'a'

try:


print user_id

except (TypeError, ValueError):

print "Oops!"


● If you’re catching multiple exceptions in python 2.x, you must enclose them in parentheses

● If you do not, the second argument to except is treated as a reference to the exception object. E.g. this may look familiar:except Exception, e

● This problem goes away in python 3.x as except Exception, e is not a valid syntax. It is replaced byexcept Exception as e


7. So long!

So long!

x = set([type(1), type(1L), type(1.0)])

y = set([1.__class__, 1L.__class__, 1.0.__class__])

print x == y

(a) True(b) False(c) TypeError(d) None of the above

So long!



print x == y

(a) True(b) False(c) TypeError(d) None of the above


^

SyntaxError: invalid syntax

It is a tokenization issue!

The . is parsed as the beginning of the fractional part of a floating point number. When it encounters something other than a number, it will throw an error.

So long!

So long!


y = set([(1).__class__, 1L.__class__, 1.0.__class__])

print x == y

(a) True(b) False(c) TypeError

(d) None of the above

8. Zilch

Zilch

z = False

i = []

l = 0,

c = None

h = {}

print any((z, i, l, c, h))

(a) True(b) False(c) (False, False, False, False, False)

(d) None of the above

When creating a tuple, in all cases except the empty tuple, the comma is the important thing.

Parentheses are only required for other syntactic reasons: to distinguish a tuple from a set of function arguments, operator precedence, or to allow line breaks.

Zilch

https://docs.python.org/2/tutorial/datastructures.html#tuples-and-sequences



9. It’s 2016!

It’s 2016!

print 2,016 * 2,016(a) 4064256(b) 4,064,256(c) 4 256(d) 2 32 16(e) None of the above

It’s 2016!

print 2,016 * 2,016(a) 4064256(b) 4,064,256(c) 4 256(d) 2 32 16(e) None of the above

2 28 14

The 0 is an outdated prefix that Python 2.x uses to represent Octal numbers.

In Python 3, you must write: 0o16 instead.

It’s 2016!

10. Finally

Finally

def find_inverse(n):

inverse = 1

try:

inverse = 1 / k

finally:

return inverse

fi = find_inverse

print fi(-1) + fi(0) + fi(1)

(a) 0(b) 1(c) 3(d) raises ZeroDivisionError(e) raises NameError(f) None of the above

Finally

return in finally will swallow the actual return value in the try block

return in finally will swallow the exception

Finally

References

● How to format code within a presentation

● Python Puzzlers by Tendayi Mawushe at PyCon Ireland 2010

● Python Puzzlers by Alan Pierce at Khan Academy

● Python Epiphanies by Stuart Williams at PyCon Montreal 2015

● Python Interview Questions

● Python Gotchas

● Raymon Hettinger’s twitter feed @raymondh

http://www.slideshare.net/tendayim/python-puzzlers




https://speakerdeck.com/alangpierce/python-puzzlers

https://speakerdeck.com/alangpierce/python-puzzlers

https://www.youtube.com/watch?v=Ug0iDjbMPVg

https://www.youtube.com/watch?v=Ug0iDjbMPVg

http://www.toptal.com/python/interview-questions

http://www.toptal.com/python/interview-questions

http://docs.python-guide.org/en/latest/writing/gotchas/



https://twitter.com/raymondh

Python Puzzlers - 2016 Edition

Software

Transcript of Python Puzzlers - 2016 Edition