 - [Python Garbage Collection][1]
 - [`gc` module docs][2]
 - [Details on Garbage Collection for Python][3]

There&#39;s no definitive resource on how Python does its garbage collection (other than the source code itself), but those 3 links should give you a pretty good idea.

## Update ##

The source is actually pretty helpful. How much you get out of it depends on how well you read C, but the comments are actually very helpful. Skip down to the [`collect()` function](https://github.com/python/cpython/blob/v3.7.0/Modules/gcmodule.c#L796) and the comments explain the process well (albeit in very technical terms).

  [1]: http://www.digi.com/wiki/developer/index.php/Python_Garbage_Collection
  [2]: http://docs.python.org/library/gc.html
  [3]: http://arctrix.com/nas/python/gc/

How can I use multiple file extensions within one group using `OpenFileDialog`?
I have `Filter = &quot;BMP|*.bmp|GIF|*.gif|JPG|*.jpg|PNG|*.png|TIFF|*.tiff&quot;`
and I want to create groups so JPG are *.jpg and *.jpeg, TIFF are *.tif and *.tiff and also &#39;All graphic types&#39;? How can I do that?

Multiple file extensions in OpenFileDialog

I have a collection `T`, with 2 fields: `Grade1` and `Grade2`, and I want to select those with condition `Grade1 &gt; Grade2`, how can I get a query like in MySQL?

    Select * from T Where Grade1 &gt; Grade2



MongoDb query condition on comparing 2 fields

I&#39;m looking for documents that describes in details how python garbage collection works.

 I&#39;m interested what is done in which step. What objects are in these 3 collections? What kinds of objects are deleted in each step? What algorithm is used for reference cycles finding?

Background: I&#39;m implementing some searches that have to finish in small amount of time. When the garbage collector starts collecting the oldest generation, it is &quot;much&quot; slower than in other cases. It took more time than it is intended for searches. I&#39;m looking how to predict when it will collect oldest generation and how long it will take.

It is easy to predict when it will collect oldest generation with `get_count()` and `get_threshold()`. That also can be manipulated with `set_threshold()`. But I don&#39;t see how easy to decide is it better to make `collect()` by force or wait for scheduled collection.

Python garbage collector documentation

I'm looking for documents that describes in details how python garbage collection works.
I'm interested what is done in which step. What objects are in these 3 collections? What kinds of objects are deleted in each step? What algorithm is used for reference cycles finding?
Background: I'm implementing some searches that have to finish in small amount of time. When the garbage collector starts collecting the oldest generation, it is "much" slower than in other cases. It took more time than it is intended for searches. I'm looking how to predict when it will collect oldest generation and how long it will take.
It is easy to predict when it will collect oldest generation with <code>get_count()</code> and <code>get_threshold()</code>. That also can be manipulated with <code>set_threshold()</code>. But I don't see how easy to decide is it better to make <code>collect()</code> by force or wait for scheduled collection.

It looks like you can&#39;t use exec in a function that has a subfunction...

Anyone know why this Python code doesn&#39;t work?  I get an error at the exec in test2.  Also, I know exec&#39;s aren&#39;t good style, but trust me, I&#39;m using exec for an appropriate reason. I wouldn&#39;t use it otherwise.

    #!/usr/bin/env python
    #
    
    def test1():
        exec(&#39;print &quot;hi from test1&quot;&#39;)
    
    test1()
    
    def test2():
        &quot;&quot;&quot;Test with a subfunction.&quot;&quot;&quot;
        exec(&#39;print &quot;hi from test2&quot;&#39;)
        def subfunction():
            return True
    
    test2()

EDIT: I narrowed down the bug to having a function in a subfunction. It has nothing to do with the raise keyword.

Why doesn&#39;t exec work in a function with a subfunction?

I notice there is a comparison operator `is not`. Should I literally translate it into 

    != 

instead of 

    == not


python &#39;is not&#39; operator

Let&#39;s say I have a file. How do I write &quot;hello&quot; TAB &quot;alex&quot;?


How do I write a &quot;tab&quot; in Python?

I was trying to install Python packages a system I recently gained access to. I was trying to take advantage of Python&#39;s relatively new [per user site-packages directory][1], and the new option `--user`. (The option is [currently undocumented][2], however it exists for Python 2.6+; you can see the help by running `python setup.py install --help`.)

When I tried running

    python setup.py install --user

on any package I downloaded, I always got the following error:

    error: can&#39;t combine user with with prefix/exec_prefix/home or install_(plat)base

The error was extremely perplexing because, as you can see, I wasn&#39;t providing the `--prefix`, `--exec-prefix`, `--install-base`, or `--install-platbase` flags as command line options. I wasted a lot of time trying to figure out what the problem was. I document my answer below, in hopes to spare some other poor soul a few hours of [yak shaving][3].


  [1]: http://docs.python.org/whatsnew/2.6.html#pep-370-per-user-site-packages-directory
  [2]: http://bugs.python.org/issue10745
  [3]: http://en.wiktionary.org/wiki/yak_shaving

Combine --user with --prefix error with setup.py install

I have a Python program that spawns many threads, runs 4 at a time, and each performs an expensive operation. Pseudocode:

    for object in list:
        t = Thread(target=process, args=(object))
        # if fewer than 4 threads are currently running, t.start(). Otherwise, add t to queue

But when the program is run, Activity Monitor in OS X shows that 1 of the 4 logical cores is at 100% and the others are at nearly 0. Obviously I can&#39;t force the OS to do anything but I&#39;ve never had to pay attention to performance in multi-threaded code like this before so I was wondering if I&#39;m just missing or misunderstanding something.

Thanks.


Python threads all executing on a single core

It appears that `System.Timers.Timer` instances are kept alive by some mechanism, but `System.Threading.Timer` instances are not.

Sample program, with a periodic `System.Threading.Timer` and auto-reset `System.Timers.Timer`:

    class Program
    {
      static void Main(string[] args)
      {
        var timer1 = new System.Threading.Timer(
          _ =&gt; Console.WriteLine(&quot;Stayin alive (1)...&quot;),
          null,
          0,
          400);
            
        var timer2 = new System.Timers.Timer
        {
          Interval = 400,
          AutoReset = true
        };
        timer2.Elapsed += (_, __) =&gt; Console.WriteLine(&quot;Stayin alive (2)...&quot;);
        timer2.Enabled = true;

        System.Threading.Thread.Sleep(2000);

        Console.WriteLine(&quot;Invoking GC.Collect...&quot;);
        GC.Collect();

        Console.ReadKey();
      }
    }

When I run this program (.NET 4.0 Client, Release, outside the debugger), only the `System.Threading.Timer` is GC&#39;ed:

    Stayin alive (1)...
    Stayin alive (1)...
    Stayin alive (2)...
    Stayin alive (1)...
    Stayin alive (2)...
    Stayin alive (1)...
    Stayin alive (2)...
    Stayin alive (1)...
    Stayin alive (2)...
    Invoking GC.Collect...
    Stayin alive (2)...
    Stayin alive (2)...
    Stayin alive (2)...
    Stayin alive (2)...
    Stayin alive (2)...
    Stayin alive (2)...
    Stayin alive (2)...
    Stayin alive (2)...
    Stayin alive (2)...


**EDIT**: I&#39;ve accepted John&#39;s answer below, but I wanted to expound on it a bit.

When running the sample program above (with a breakpoint at `Sleep`), here&#39;s the state of the objects in question and the `GCHandle` table:

    !dso
    OS Thread Id: 0x838 (2104)
    ESP/REG  Object   Name
    0012F03C 00c2bee4 System.Object[]    (System.String[])
    0012F040 00c2bfb0 System.Timers.Timer
    0012F17C 00c2bee4 System.Object[]    (System.String[])
    0012F184 00c2c034 System.Threading.Timer
    0012F3A8 00c2bf30 System.Threading.TimerCallback
    0012F3AC 00c2c008 System.Timers.ElapsedEventHandler
    0012F3BC 00c2bfb0 System.Timers.Timer
    0012F3C0 00c2bfb0 System.Timers.Timer
    0012F3C4 00c2bfb0 System.Timers.Timer
    0012F3C8 00c2bf50 System.Threading.Timer
    0012F3CC 00c2bfb0 System.Timers.Timer
    0012F3D0 00c2bfb0 System.Timers.Timer
    0012F3D4 00c2bf50 System.Threading.Timer
    0012F3D8 00c2bee4 System.Object[]    (System.String[])
    0012F4C4 00c2bee4 System.Object[]    (System.String[])
    0012F66C 00c2bee4 System.Object[]    (System.String[])
    0012F6A0 00c2bee4 System.Object[]    (System.String[])

    !gcroot -nostacks 00c2bf50

    !gcroot -nostacks 00c2c034
    DOMAIN(0015DC38):HANDLE(Strong):9911c0:Root:  00c2c05c(System.Threading._TimerCallback)-&gt;
      00c2bfe8(System.Threading.TimerCallback)-&gt;
      00c2bfb0(System.Timers.Timer)-&gt;
      00c2c034(System.Threading.Timer)

    !gchandles
    GC Handle Statistics:
    Strong Handles:       22
    Pinned Handles:       5
    Async Pinned Handles: 0
    Ref Count Handles:    0
    Weak Long Handles:    0
    Weak Short Handles:   0
    Other Handles:        0
    Statistics:
          MT    Count    TotalSize Class Name
    7aa132b4        1           12 System.Diagnostics.TraceListenerCollection
    79b9f720        1           12 System.Object
    79ba1c50        1           28 System.SharedStatics
    79ba37a8        1           36 System.Security.PermissionSet
    79baa940        2           40 System.Threading._TimerCallback
    79b9ff20        1           84 System.ExecutionEngineException
    79b9fed4        1           84 System.StackOverflowException
    79b9fe88        1           84 System.OutOfMemoryException
    79b9fd44        1           84 System.Exception
    7aa131b0        2           96 System.Diagnostics.DefaultTraceListener
    79ba1000        1          112 System.AppDomain
    79ba0104        3          144 System.Threading.Thread
    79b9ff6c        2          168 System.Threading.ThreadAbortException
    79b56d60        9        17128 System.Object[]
    Total 27 objects

As John pointed out in his answer, both timers register their callback (`System.Threading._TimerCallback`) in the `GCHandle` table. As Hans pointed out in his comment, the `state` parameter is also kept alive when this is done.

As John pointed out, the reason `System.Timers.Timer` is kept alive is because it is referenced by the callback (it is passed as the `state` parameter to the inner `System.Threading.Timer`); likewise, the reason our `System.Threading.Timer` is GC&#39;ed is because it is *not* referenced by its callback.

Adding an explicit reference to `timer1`&#39;s callback (e.g., `Console.WriteLine(&quot;Stayin alive (&quot; + timer1.GetType().FullName + &quot;)&quot;)`) is sufficient to prevent GC.

Using the single-parameter constructor on `System.Threading.Timer` also works, because the timer will then reference itself as the `state` parameter. The following code keeps both timers alive after the GC, since they are each referenced by their callback from the `GCHandle` table:

    class Program
    {
      static void Main(string[] args)
      {
        System.Threading.Timer timer1 = null;
        timer1 = new System.Threading.Timer(_ =&gt; Console.WriteLine(&quot;Stayin alive (1)...&quot;));
        timer1.Change(0, 400);

        var timer2 = new System.Timers.Timer
        {
          Interval = 400,
          AutoReset = true
        };
        timer2.Elapsed += (_, __) =&gt; Console.WriteLine(&quot;Stayin alive (2)...&quot;);
        timer2.Enabled = true;

        System.Threading.Thread.Sleep(2000);

        Console.WriteLine(&quot;Invoking GC.Collect...&quot;);
        GC.Collect();

        Console.ReadKey();
      }
    }


Why does a System.Timers.Timer survive GC but not System.Threading.Timer?

If you see the Android logs, you may see a lot of those things.

What do they mean, knowing those may help us doing better memory allocations.

Example:

     28470               dalvikvm  D  GC_FOR_MALLOC freed 665 objects / 239992 bytes in 71ms
     28470               dalvikvm  D  GC_FOR_MALLOC freed 673 objects / 240288 bytes in 87ms
     21940               dalvikvm  D  GC_EXPLICIT freed 4802 objects / 185320 bytes in 78ms
     28470               dalvikvm  D  GC_FOR_MALLOC freed 666 objects / 240536 bytes in 63ms



What do GC_FOR_MALLOC, GC_EXPLICIT, and other GC_* mean in Android Logcat?

I saw some post about implement GC in C and some people said it&#39;s impossible to do it because C is weakly typed. I want to know how to implement GC in C++.

I want some general idea about how to do it. Thank you very much!

This is a Bloomberg interview question my friend told me. He did badly at that time. We want to know your ideas about this. 

How to implement garbage collection in C++

Short question,

I saw in some old code where a `ByteArrayInputStream` was created like:

    new BufferedReader(new InputStreamReader(new ByteArrayInputStream(somebytes)));

And then the `BufferedReader` is used to read out `somebytes` line by line.  
All working fine, but I noticed that the `BufferedReader` is never closed.  
This is all working in a long running websphere application, the `somebytes` are not terrible big (200k most), it is only invoked a few times a week and we&#39;re not experiencing any apparent memory leaks. So I expect that all the objects are successfully garbage collected.

I always (once) learned that input/output streams need to be closed, in a `finally` statement. Are `ByteStreams` the exception to this rule?

kind regards
Jeroen.

Do I need to close a ByteArrayInputStream?

I need to read a large text file of around 5-6 GB line by line using Java.

How can I do this quickly?


Content Type	Original Author	Original Content on Stackoverflow
Question	Ante	View Question on Stackoverflow
Solution 1 - Python	Rafe Kettler	View Answer on Stackoverflow

Python garbage collector documentation

Python Problem Overview

Python Solutions

Solution 1 - Python

Update

MongoDb query condition on comparing 2 fields

Multiple file extensions in OpenFileDialog

Attributions