Skip to main content

How to Add Memory Leaks to Python

One of our greatest bragging rights is the lack of memory management in our Python code and the wonder of garbage collection, so when we find a way to get a memory leak in Python, it should be made well known. I don't know if this is already known, or not. In actuality, these situations are known as reference leaks, sometimes, and they are cases where we forget to remove a reference to an object we don't want to keep around anymore. The following session will cause this problem.


Python 2.4.3 (#2, Oct 6 2006, 07:52:30)
[GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def f():
... for i in xrange(100):
... yield i
... raise Exception("oh no!")
...
>>> [x for x in f()]
Traceback (most recent call last):
File "", line 1, in ?
File "", line 4, in f
Exception: oh no!
>>> globals()['_[1]']
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
>>>


Now, there is a global that will sit around referencing a potentially very large list and we won't be aware of it. However, it will be overwritten if another list comprehension is run in the same scope, which will be removed if the new LC is successful, and if we do our LCs in functions, the local will be cleaned up on return or raise. Of course, you can always just pass a generator expression to list() and avoid the problem entirely.

Just keep an eye out, if you build global constants with list comprehensions.

Comments

Kevin said…
Doesn't '_' only exist in the interactive interpreter?
Manuel said…
"_" in the interactive interpreter is completely different than "_[1]" in globals

<pre>

    Python 2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit (Intel)]
    IPython 0.8.0 -- An enhanced Interactive Python.
    
    In [1]: def f():
       ...:     for i in xrange(10):
       ...:         yield i
       ...:     raise Exception("oh no!")
       ...:
    
    In [2]: [x for x in f()]
    ---------------------------------------------------------------------------
    <type 'exceptions.Exception'>             Traceback (most recent call last)
    ...
    <type 'exceptions.Exception'>: oh no!
    
    In [3]: 5
    Out[3]: 5
    
    In [4]: _
    Out[4]: 5
    
    In [5]: globals()['_[1]']
    Out[5]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

</pre>
PJE said…
Of course, if your code raises an exception at the global level, you're going to have other problems besides the left-over list...

Popular posts from this blog

Interrupting Coders Isn’t So Bad

Here’s a hot take: disrupting coders isn’t all that bad.

Some disruptions are certainly bad but they usually aren’t. The coder community has overblown the impact. A disruption can be a good thing. How harmful disruption might be a symptom of other problems.

There are different kinds of disruptions. They are caused by other coders on your team, managers and other non-coders, or meetings throughout the day.

The easiest example to debunk is a question from a fellow developer. Imagine someone walks over to your desk or they ping you on Slack, because they have “one quick question.” Do you get annoyed at the interruption when you were in the middle of something important? You help out your teammate quickly and get back to work, trying to pick up where you left off. That’s a kind of interruption we complain about frequently, but I’m not convinced this is all that bad.

You are being disrupted but your team, of which you are only one member of the whole unit, is working smoothly. You unstuck …

CARDIAC: The Cardboard Computer

I am just so excited about this.


CARDIAC. The Cardboard Computer. How cool is that? This piece of history is amazing and better than that: it is extremely accessible. This fantastic design was built in 1969 by David Hagelbarger at Bell Labs to explain what computers were to those who would otherwise have no exposure to them. Miraculously, the CARDIAC (CARDboard Interactive Aid to Computation) was able to actually function as a slow and rudimentary computer. 
One of the most fascinating aspects of this gem is that at the time of its publication the scope it was able to demonstrate was actually useful in explaining what a computer was. Could you imagine trying to explain computers today with anything close to the CARDIAC?

It had 100 memory locations and only ten instructions. The memory held signed 3-digit numbers (-999 through 999) and instructions could be encoded such that the first digit was the instruction and the second two digits were the address of memory to operate on. The only re…

How To Care If BSD, MIT, or GPL Licenses Are Used

The two recent posts about some individuals' choice of GPL versus others' preference for BSD and MIT style licensing has caused a lot of debate and response. I've seen everything as an interesting combination of very important topics being taken far too seriously and far too personally. All involved need to take a few steps back.

For the uninitiated and as a clarifier for the initiated, we're dealing with (basically) three categories of licensing when someone releases software (and/or its code):
Closed Source. Easiest to explain, because you just get nothing.GPL. If you get the software, you get the source code, you get to change it, and anything you combine it with must be under the same terms.MIT and BSD. If you get the software, you might get the source code, you get to change it, and you have no obligations about anything else you combine it with.The situation gets stickier when we look at those combinations and the transitions between them.

Use GPL code with Closed S…