cmishra - 4 months ago 22

Python Question

I'm trying to implement a

`StreamingMedian`

`get_median()`

`MinHeap`

`MaxHeap`

`heapq`

I've run into a very odd bug though. For some reason when I run the following commands:

`print("Before streaming medians", MinHeap(), sep="\t") # is empty`

b = StreamingMedian()

b.add_val(5)

b.add_val(100)

assert b.get_median() == 52.5

print("After streaming medians, for MaxHeap", MaxHeap(), sep='\t') # is empty

print("After streaming medians, for MinHeap", MinHeap(), sep='\t') # should be empty

print("After streaming medians, for MinHeap with input",

MinHeap([]), sep="\t") # is empty

I get the following outputs:

`Before streaming medians []`

After streaming medians, for MaxHeap []

After streaming medians, for MinHeap [100]

After streaming medians, for MinHeap with input []

The class implementations can be found below. I'm running this on Python 3.5.2 :: Anaconda custom (64-bit).

`import heapq`

class MinHeap(object):

def __init__(self, l=[]):

self.heap = l

heapq.heapify(l)

def peek(self):

return self.heap[0]

def pop(self):

return heapq.heappop(self.heap)

def push(self, x):

heapq.heappush(self.heap, x)

def pushpop(self, x):

return heapq.heappushpop(self.heap, x)

def replace(self, x):

return heapq.heapreplace(self.heap, x)

def __len__(self):

return len(self.heap)

def __str__(self):

return str(self.heap)

class MaxHeap(MinHeap):

def _invert_sign(self, l):

return [-1 * a for a in l]

def __init__(self, l=[]):

super().__init__(self._invert_sign(l))

def push(self, x):

super().push(-1 * x)

def pushpop(self, x):

return super().pushpop(-1 * x)

def replace(self, x):

return super().replace(-1 * x)

def pop(self):

return -1 * super().pop()

def peek(self):

return -1 * super().peek()

def __str__(self):

return str(self._invert_sign(self.heap))

class StreamingMedian():

def __init__(self):

self.min_heap = MinHeap()

self.max_heap = MaxHeap()

def get_median(self):

min_heap_has_x_more = len(self.min_heap) - len(self.max_heap)

if min_heap_has_x_more > 0:

return self.min_heap.peek()

elif min_heap_has_x_more < 0:

return self.max_heap.peek()

else:

return (self.min_heap.peek() + self.max_heap.peek())/2

def add_val(self, x):

if len(self.min_heap) + len(self.max_heap) == 0:

self.max_heap.push(x)

else:

med = self.get_median()

if x > med:

self.min_heap.push(x)

self._ensure_balance()

elif x < med:

self.max_heap.push(x)

self._ensure_balance()

else:

self.max_heap.push(x)

self._ensure_balance()

def _ensure_balance(self):

size_diff = len(self.min_heap) - len(self.max_heap)

if abs(size_diff) > 1:

if size_diff > 0: # min_heap has 2 more elements

self.max_heap.push(self.min_heap.pop())

else: # max_heap has 2 more elements

self.min_heap.push(self.max_heap.pop())

assert abs(len(self.min_heap) - len(self.max_heap)) < 2

print("Before streaming medians", MinHeap(), sep="\t")

b = StreamingMedian()

b.add_val(5)

b.add_val(100)

assert b.get_median() == 52.5

print("After streaming medians, for MaxHeap", MaxHeap(), sep='\t') # is empty

print("After streaming medians, for MinHeap", MinHeap(), sep='\t') # should be empty

print("After streaming medians, for MinHeap with input", MinHeap([]), sep="\t") # is empty

Answer

The problem was occurring because of how default arguments are calculated in python. They are computed the first time a function is called and afterwards the function is used with the originally calculated value. If the default value is mutable (as a list is) then this poses a problem.

So what was happening for the `MinHeap`

was:

`MinHeap`

initially created and the`l`

parameter default argument was assigned to a memory address.`StreamingMedian`

modifies`MinHeap`

's`self.heap`

which is the same as what`l`

points to.- When
`MinHeap`

is called again,`l`

already has a memory address and this memory address is used again and binded with`self.heap`

.

This doesn't happen for `MaxHeap`

because:

`MaxHeap`

initially created and the`l`

parameter default argument was assigned to a memory address.`_invert_sign`

creates a new list which is assigned to`self.heap`

. The default argument`l`

is never modified.- When
`MaxHeap`

is initialized again,`l`

already has a memory address and is used again, but it was never modified and another copy is constructed so it will never be modified.

Instead of:

```
class MinHeap(object):
def __init__(self, l=[]):
self.heap = l
heapq.heapify(l)
```

We should use:

```
class MinHeap(object):
def __init__(self, l=None):
if l is None:
l = []
self.heap = l
heapq.heapify(l)
```

A similar change should be made for `MaxHeap`