[Ideone][1] supports Python 2.6 and Python 3


  [1]: http://ideone.com

I recently came across Python 3 interpreter at [CompileOnline][1].

  [1]: http://www.compileonline.com/execute_python3_online.php

Having a chain of &quot;instanceof&quot; operations is considered a &quot;code smell&quot;.  The standard answer is &quot;use polymorphism&quot;.  How would I do it in this case?

There are a number of subclasses of a base class; none of them are under my control.  An analogous situation would be with the Java classes Integer, Double, BigDecimal etc.

    if (obj instanceof Integer) {NumberStuff.handle((Integer)obj);}
    else if (obj instanceof BigDecimal) {BigDecimalStuff.handle((BigDecimal)obj);}
    else if (obj instanceof Double) {DoubleStuff.handle((Double)obj);}

I do have control over NumberStuff and so on.

I don&#39;t want to use many lines of code where a few lines would do.  (Sometimes I make a HashMap mapping Integer.class to an instance of IntegerStuff, BigDecimal.class to an instance of BigDecimalStuff etc.  But today I want something simpler.)

I&#39;d like something as simple as this:

    public static handle(Integer num) { ... }
    public static handle(BigDecimal num) { ... }

But Java just doesn&#39;t work that way.

I&#39;d like to use static methods when formatting.  The things I&#39;m formatting are composite, where a Thing1 can contain an array Thing2s and a Thing2 can contain an array of Thing1s.  I had a problem when I implemented my formatters like this:

    class Thing1Formatter {
      private static Thing2Formatter thing2Formatter = new Thing2Formatter();
      public format(Thing thing) {
          thing2Formatter.format(thing.innerThing2);
      }
    }
    class Thing2Formatter {
      private static Thing1Formatter thing1Formatter = new Thing1Formatter();
      public format(Thing2 thing) {
          thing1Formatter.format(thing.innerThing1);
      }
    }

Yes, I know the HashMap and a bit more code can fix that too.  But the &quot;instanceof&quot; seems so readable and maintainable by comparison.  Is there anything simple but not smelly?

Note added 5/10/2010:

It turns out that new subclasses will probably be added in the future, and my existing code will have to handle them gracefully.  The HashMap on Class won&#39;t work in that case because the Class won&#39;t be found.  A chain of if statements, starting with the most specific and ending with the most general, is probably the best after all:

    if (obj instanceof SubClass1) {
        // Handle all the methods and properties of SubClass1
    } else if (obj instanceof SubClass2) {
        // Handle all the methods and properties of SubClass2
    } else if (obj instanceof Interface3) {
        // Unknown class but it implements Interface3
        // so handle those methods and properties
    } else if (obj instanceof Interface4) {
        // likewise.  May want to also handle case of
        // object that implements both interfaces.
    } else {
        // New (unknown) subclass; do what I can with the base class
    }


Avoiding instanceof in Java

I am running remote audio-file-fetching and audio file playback operations in a background thread using `AsyncTask`. A `Cancellable` progress bar is shown for the time the fetch operation runs. 

I want to cancel/abort the `AsyncTask` run when the user cancels (decides against) the operation. What is the ideal way to handle such a case?

Ideal way to cancel an executing AsyncTask

Is there an online interpreter like http://codepad.org/ or http://www.trypython.org/ which uses Python 3?

**Answer**

Since the question is closed, I give another answer here.

[Wandbox](http://melpon.org/wandbox/) offers online REPLs for many languages, including Python 2.x and 3.x, C++ and Java.

Python 3 Online Interpreter / Shell

Is there an online interpreter like <a href="http://codepad.org/" target="_blank" rel="noopener noreferrer">http://codepad.org/</a> or <a href="http://www.trypython.org/" target="_blank" rel="noopener noreferrer">http://www.trypython.org/</a> which uses Python 3?
Answer
Since the question is closed, I give another answer here.
<a href="http://melpon.org/wandbox/" target="_blank" rel="noopener noreferrer">Wandbox</a> offers online REPLs for many languages, including Python 2.x and 3.x, C++ and Java.

&gt; **Possible Duplicate:**  
&gt; [Unexpected feature in a Python list of lists](https://stackoverflow.com/questions/240178/unexpected-feature-in-a-python-list-of-lists)  

&lt;!-- End of automatically inserted text --&gt;

So I am relatively new to Python and I am having trouble working with 2D Lists.

Here&#39;s my code:

    data = [[None]*5]*5
    data[0][0] = &#39;Cell A1&#39;
    print data

and here is the output (formatted for readability):

    [[&#39;Cell A1&#39;, None, None, None, None],
     [&#39;Cell A1&#39;, None, None, None, None],
     [&#39;Cell A1&#39;, None, None, None, None],
     [&#39;Cell A1&#39;, None, None, None, None],
     [&#39;Cell A1&#39;, None, None, None, None]]

Why does every row get assigned the value?

2D list has weird behavor when trying to modify a single value

I have an *uniform* list of objects in python:

    class myClass(object):
        def __init__(self, attr):
            self.attr = attr
            self.other = None

    objs = [myClass (i) for i in range(10)]

Now I want to extract a list with some attribute of that class (let&#39;s say attr), in order to pass it so some function (for plotting that data for example)

What is the pythonic way of doing it,

    attr=[o.attr for o in objsm]
?

Maybe derive list and add a method to it, so I can use some idiom like

    objs.getattribute(&quot;attr&quot;)
?

Extract list of attributes from list of objects in python

From [Building Skills in Python][1]:

&gt; A file name like `exercise_1.py` is better than the name `exercise-1.py`. We can run both programs equally well from the command line, but the name with the hyphen limits our ability to write larger and more sophisticated programs.

Why is this?


  [1]: http://homepage.mac.com/s_lott/books/python/BuildingSkillsinPython.pdf

Why are underscores better than hyphens for file names?

I&#39;m trying this code:

    import sqlite
    
    connection = sqlite.connect(&#39;cache.db&#39;)
    cur = connection.cursor()
    cur.execute(&#39;&#39;&#39;create table item
      (id integer primary key, itemno text unique,
            scancode text, descr text, price real)&#39;&#39;&#39;)
    
    connection.commit()
    cur.close()

I&#39;m catching this exception:

    Traceback (most recent call last):
      File &quot;cache_storage.py&quot;, line 7, in &lt;module&gt;
        scancode text, descr text, price real)&#39;&#39;&#39;)
      File &quot;/usr/lib/python2.6/dist-packages/sqlite/main.py&quot;, line 237, in execute
        self.con._begin()
      File &quot;/usr/lib/python2.6/dist-packages/sqlite/main.py&quot;, line 503, in _begin
        self.db.execute(&quot;BEGIN&quot;)
    _sqlite.OperationalError: database is locked

Permissions for cache.db are ok. Any ideas?

Python SQLite: database is locked

I&#39;m writing a slideshow program with Tkinter, but I don&#39;t know how to change the background color to black instead of the standard light gray. How can this be done?

    import os, sys
    import Tkinter
    import Image, ImageTk
    import time
    
    root = Tkinter.Tk()
    w, h = root.winfo_screenwidth(), root.winfo_screenheight()
    root.overrideredirect(1)
    root.geometry(&quot;%dx%d+0+0&quot; % (w, h))
    root.focus_set()
    root.bind(&quot;&lt;Escape&gt;&quot;, lambda e: e.widget.quit())
    image = Image.open(image_path+f)
    tkpi = ImageTk.PhotoImage(image)        
    label_image = Tkinter.Label(root, image=tkpi)
    label_image.place(x=0,y=0,width=w,height=h)
    root.mainloop(0)

Background color for Tk in Python

In shell scripts, what is the difference between `$@` and `$*`?

Which one is the preferred way to get the script arguments?

Are there differences between the different shell interpreters about this?

What is the difference between $@ and $* in shell scripts?

I would like to define some aliases in fish. Apparently it should be possible to define them in 

    ~/.config/fish/functions

but they don&#39;t get auto loaded when I restart the shell. Any ideas?


Define an alias in fish shell

I would like to join the result of `ls -1` into one line and delimit it with whatever i want.

Are there any standard Linux commands I can use to achieve this?

How to join multiple lines of file names into one with custom delimiter?

I can open a shell by using the :shell command in Vim, however I can&#39;t edit a file and at the same time use the shell.

Is there any way to split Vim in many Windows (or tabs), and have a shell opened in one of them?

How can I open a Shell inside a Vim Window?

I would like to generate a random filename in unix shell (say tcshell). The filename should consist of random 32 hex letters, e.g.:

    c7fdfc8f409c548a10a0a89a791417c5

(to which I will add whatever is neccesary). The point is being able to do it only in shell without resorting to a program.





Generate a random filename in unix shell

Here&#39;s my code:

    import urllib2.request
    
    response = urllib2.urlopen(&quot;http://www.google.com&quot;)
    html = response.read()
    print(html)

Any help?

Import error: No module name urllib2

Is there a way to generate random letters in Python (like random.randint but for letters)? The range functionality of random.randint would be nice but having a generator that just outputs a random letter would be better than nothing.

Generate a random letter in Python

In Python, is there a way to ping a server through ICMP and return TRUE if the server responds, or FALSE if there is no response?

Pinging servers in Python

I am getting the following error:

    WindowsError: [Error 2] The system cannot find the file specified

My code is:

    subprocess.call([&quot;&lt;&lt;executable file found in PATH&gt;&gt;&quot;])

Windows 7, 64 bit. Python 3.x latest, stable.

Any ideas?

Thanks,

Windows can&#39;t find the file on subprocess.call()

Function Annotations: [PEP-3107][1]

I ran across a snippet of code demonstrating Python3&#39;s function annotations.  The concept is simple but I can&#39;t think of why these were implemented in Python3 or any good uses for them.  Perhaps SO can enlighten me?

How it works:

    def foo(a: &#39;x&#39;, b: 5 + 6, c: list) -&gt; max(2, 9):
        ... function body ...
    
Everything following the colon after an argument is an &#39;annotation&#39;, and the information following the `-&gt;` is an annotation for the function&#39;s return value.  

foo.func_annotations would return a dictionary:

    {&#39;a&#39;: &#39;x&#39;,
     &#39;b&#39;: 11,
     &#39;c&#39;: list,
     &#39;return&#39;: 9}

What&#39;s the significance of having this available?

  [1]: http://www.python.org/dev/peps/pep-3107/


What are good uses for Python3&#39;s &quot;Function Annotations&quot;?

I&#39;m trying to get a better understanding of the difference. I&#39;ve found a lot of explanations online, but they tend towards the abstract differences rather than the practical implications.

Most of my programming experiences has been with CPython (dynamic, interpreted), and Java (static, compiled). However, I understand that there are other kinds of interpreted and compiled languages. Aside from the fact that executable files can be distributed from programs written in compiled languages, are there any advantages/disadvantages to each type? Oftentimes, I hear people arguing that interpreted languages can be used interactively, but I believe that compiled languages can have interactive implementations as well, correct?

Compiled vs. Interpreted Languages

I&#39;m trying to understand the python compiler/interpreter process more clearly. Unfortunately, I have not taken a class in interpreters nor have I read much about them.

Basically, what I understand right now is that Python code from `.py` files is first compiled into python bytecode (which I assume are the `.pyc` files I see occasionally?). Next, the bytecode is compiled into machine code, a language the processor actually understands.
Pretty much, I&#39;ve read this thread https://stackoverflow.com/questions/888100/why-python-compile-the-source-to-bytecode-before-interpreting

Could somebody give me a good explanation of the whole process keeping in mind that my knowledge of compilers/interpreters is almost non-existent? Or, if that&#39;s not possible, maybe give me some resources that give quick overviews of compilers/interpreters?

Thanks

Python Compilation/Interpretation Process

I have been trying to understand the exact meaning/purpose of loading php as an apache module vs the rest.

When php is installed as an apache module, what exactly happens? For example, does reading the php-ini file happen every time the php request comes or when the php module is loaded alone?

mod_php vs cgi vs fast-cgi

How do I repeat the last command? The usual keys: Up, Ctrl+Up, Alt-p don&#39;t work. They produce nonsensical characters.

    (ve)[kakarukeys@localhost ve]$ python
    Python 2.6.6 (r266:84292, Nov 15 2010, 21:48:32) 
    [GCC 4.4.4 20100630 (Red Hat 4.4.4-10)] on linux2
    Type &quot;help&quot;, &quot;copyright&quot;, &quot;credits&quot; or &quot;license&quot; for more information.
    &gt;&gt;&gt; print &quot;hello world&quot;
    hello world
    &gt;&gt;&gt; ^[[A
      File &quot;&lt;stdin&gt;&quot;, line 1
        ^
    SyntaxError: invalid syntax
    &gt;&gt;&gt; ^[[1;5A
      File &quot;&lt;stdin&gt;&quot;, line 1
        [1;5A
        ^
    SyntaxError: invalid syntax
    &gt;&gt;&gt; ^[p
      File &quot;&lt;stdin&gt;&quot;, line 1
        p
        ^
    SyntaxError: invalid syntax
    &gt;&gt;&gt; 


How to repeat last command in python interpreter shell?

I want to run a Python script from a Python script with `subprocess`, and I wish to do it using the same interpreter for each of them.

I&#39;m using virtualenv, so I&#39;d like to do something like:

    subprocess.Popen(&#39;%s script.py&#39; % python_bin)

How do I get `python_bin`?

It should be `/usr/bin/python` outside a virtualenv, and `/path/to/env/bin/python` in a virtualenv.


Content Type	Original Author	Original Content on Stackoverflow
Question	deamon	View Question on Stackoverflow
Solution 1 - Python	kuszi	View Answer on Stackoverflow
Solution 2 - Python	Abbas	View Answer on Stackoverflow

Python 3 Online Interpreter / Shell

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

Ideal way to cancel an executing AsyncTask

Avoiding instanceof in Java

Attributions