    &gt;&gt;&gt; rr,tt = zip(*[(i*10, i*12) for i in xrange(4)])
    &gt;&gt;&gt; rr
    (0, 10, 20, 30)
    &gt;&gt;&gt; tt
    (0, 12, 24, 36)

Creating two comprehensions list is better (at least for long lists). Be aware that, the **best voted answer is slower** can be even slower than traditional for loops. ***List comprehensions are faster and clearer***.

    python -m timeit -n 100 -s &#39;rr=[];tt = [];&#39; &#39;for i in range(500000): rr.append(i*10);tt.append(i*12)&#39; 
    10 loops, best of 3: 123 msec per loop

    &gt; python -m timeit -n 100 &#39;rr,tt = zip(*[(i*10, i*12) for i in range(500000)])&#39; 
    10 loops, best of 3: 170 msec per loop

    &gt; python -m timeit -n 100 &#39;rr = [i*10 for i in range(500000)]; tt = [i*10 for i in range(500000)]&#39; 
    10 loops, best of 3: 68.5 msec per loop

It would be nice to see list comprehensionss supporting the creation of multiple lists at a time. 

# However,

***if you can take an advantage of using a traditional loop (to be precise, intermediate calculations),*** then it is possible that you will be better of with a loop (or an `iterator`/`generator` using `yield`). Here is an example:

    $ python3 -m timeit -n 100 -s &#39;rr=[];tt=[];&#39; &quot;for i in (range(1000) for x in range(10000)): tmp = list(i); rr.append(min(tmp));tt.append(max(tmp))&quot; 
    100 loops, best of 3: 314 msec per loop

    $ python3 -m timeit -n 100 &quot;rr=[min(list(i)) for i in (range(1000) for x in range(10000))];tt=[max(list(i)) for i in (range(1000) for x in range(10000))]&quot;
    100 loops, best of 3: 413 msec per loop

Of course, ***the comparison in these cases are unfair***; in the example, the code and calculations are not equivalent because in the traditional loop a temporary result is stored (see `tmp` variable). So, the list comprehension is doing much more internal operations (it calculates the tmp variable twice!, yet it is only 25% slower).



It is possible for a list comprehension to return multiple lists if the elements are lists.
So for example:

    &gt;&gt;&gt; x, y = [[] for x in range(2)]
    &gt;&gt;&gt; x
    []
    &gt;&gt;&gt; y
    []
    &gt;&gt;&gt;

The trick with `zip` function would do the job, but actually is much more simpler and readable if you just collect the results in lists with a loop.

I ran into an issue where I assigned `request.user` to a variable called `prior_user`, then essentially authenticated the user, then checked to see if `request.user != prior_user`. I expected them not to be the same and that `prior_user` should contain `AnonymousUser. To my surprise, they were the same. 

Sample code:

    prior_user = request.user   # request object, obtained froma  view
    authenticate_user(request)   # some function that authenticates
    print prior_user.username != request.user.username   # returns False i.e.they are the same!

I then discovered prior_user actually contains an instance of django.utils.functional.SimpleLazyObject so I assume it is some sort of lazy lookup type thing i.e. prior_user&#39;s value isn&#39;t looked up until actually used. Looking at the source code, I cannot confirm this.

Anyone with django experience can tell me what is going on and why it is needed? 

This leaves me a little shaken, because the usual assignment statement doesn&#39;t work the way I expect and what else within Django acts like this? Nor did I see this described in the [docs][1].

So anyone with super human knowledge of django can provide some clarity?

  [1]: https://docs.djangoproject.com/en/dev/ref/request-response/#httprequest-objects

django: Purpose of django.utils.functional.SimpleLazyObject?

The regex `/abc$/` will match an `abc` that does appear at the end of the line. How do I do the inverse?

I want to match `abc` that isn&#39;t at the end of a line.

Furthermore, I&#39;m going to be using the regex to replace strings, so I want to capture only `abc`, not anything after the string, so `/abc.+$/` doesn&#39;t work, because it would replace not only `abc` but anything after `abc` too.

What is the correct regex to use?


What is a regex to match a string NOT at the end of a line?

Is it possible to return two lists from a list comprehension? Well, this obviously doesn&#39;t work, but something like:

    rr, tt = [i*10, i*12 for i in xrange(4)]

So `rr` and `tt` both are lists with the results from `i*10` and `i*12` respectively. 
Many thanks

Possible to return two lists from a list comprehension?

Is it possible to return two lists from a list comprehension? Well, this obviously doesn't work, but something like:
<pre><code class="hljs language-ini">rr, tt = [i*10, i*12 for i in xrange(4)]
</code></pre>
So <code>rr</code> and <code>tt</code> both are lists with the results from <code>i*10</code> and <code>i*12</code> respectively.
Many thanks

Originally I wanted to ask [this question][1], but then I found it was already thought of before... 

Googling around I found this example of [extending configparser][2]. The following works with Python 3:

    $ python3
    Python 3.2.3rc2 (default, Mar 21 2012, 06:59:51) 
    [GCC 4.6.3] on linux2
    &gt;&gt;&gt; from configparser import  SafeConfigParser
    &gt;&gt;&gt; class AmritaConfigParser(SafeConfigParser):
    ...     def __init__(self):
    ...         super().__init__()
    ... 
    &gt;&gt;&gt; cfg = AmritaConfigParser()

But not with Python 2:

    &gt;&gt;&gt; class AmritaConfigParser(SafeConfigParser):
    ...       def __init__(self):
    ...           super(SafeConfigParser).init()
    ... 
    &gt;&gt;&gt; cfg = AmritaConfigParser()
    Traceback (most recent call last):
      File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
      File &quot;&lt;stdin&gt;&quot;, line 3, in __init__
    TypeError: must be type, not classob

Then I read a little bit on Python New Class vs. Old Class styles (e.g. [here][3].
And now I am wondering, I can do:

    class MyConfigParser(ConfigParser.ConfigParser):
          def Write(self, fp):
              &quot;&quot;&quot;override the module&#39;s original write funcition&quot;&quot;&quot;
              ....
          def MyWrite(self, fp):
              &quot;&quot;&quot;Define new function and inherit all others&quot;&quot;&quot;


But, shouldn&#39;t I call init? Is this in Python 2 the equivalent:

     class AmritaConfigParser(ConfigParser.SafeConfigParser):
        #def __init__(self):
        #    super().__init__() # Python3 syntax, or rather, new style class syntax ...
        #
        # is this the equivalent of the above ?	
        def __init__(self):
            ConfigParser.SafeConfigParser.__init__(self)


  [1]: https://stackoverflow.com/questions/4058400/using-colons-in-configparser-python
  [2]: http://sureshamrita.wordpress.com/2011/08/28/extending-python-configparser/ozn@yenikaro:~/Software/
  [3]: https://stackoverflow.com/questions/1713038/super-fails-with-error-typeerror-argument-1-must-be-type-not-classobj


Python extending with - using super() Python 3 vs Python 2

I have a large dictionary constructed like so:

    programs[&#39;New York&#39;] = &#39;some values...&#39; 
    programs[&#39;Port Authority of New York&#39;] = &#39;some values...&#39; 
    programs[&#39;New York City&#39;] = &#39;some values...&#39;
    ...

How can I return all elements of `programs` whose key mentions &quot;new york&quot; (case insensitive)? In the example above, I would want to get all the three items.

**EDIT:** The dictionary is quite large and expected to get larger over time.

Find dictionary items whose key matches a substring

I&#39;m trying to add items to an array in python.

I run 

    array = {}

Then, I try to add something to this array by doing:

    array.append(valueToBeInserted)

There doesn&#39;t seem to be a `.append` method for this.  How do I add items to an array?

How to declare and add items to an array in Python?

I have a date time string that I don&#39;t know how to parse it in Python.

The string is like this:

    Tue May 08 15:14:45 +0800 2012

I tried 

    datetime.strptime(&quot;Tue May 08 15:14:45 +0800 2012&quot;,&quot;%a %b %d %H:%M:%S %z %Y&quot;)

but Python raises 

    &#39;z&#39; is a bad directive in format &#39;%a %b %d %H:%M:%S %z %Y&#39;

According to Python doc:

&gt; %z	UTC offset in the form +HHMM or -HHMM (empty string if the the object is naive).

**What is the right format to parse this time string?**

Parsing time string in Python

I have:

    dictionary = {&quot;foo&quot;:12, &quot;bar&quot;:2, &quot;jim&quot;:4, &quot;bob&quot;: 17}

I want to iterate over this dictionary, but over the values instead of the keys, so I can use the values in another function.

For example, I want to test which dictionary values are greater than `6`, and then store their keys in a list. My code looks like this:

    list = []
    for c in dictionary:
        if c &gt; 6:
            list.append(dictionary[c])
    print list

and then, in a perfect world, `list` would feature all the keys whose value is greater than `6`.
However, my `for` loop is only iterating over the keys; I would like to change that to the values!

Any help is greatly appreciated.
thank you

How to filter dictionary keys based on its corresponding values

Lets suppose I have a list like this:

    mylist = [&quot;a&quot;,&quot;b&quot;,&quot;c&quot;,&quot;d&quot;]

To get the values printed along with their index I can use Python&#39;s `enumerate` function like this

    &gt;&gt;&gt; for i,j in enumerate(mylist):
    ...     print i,j
    ...
    0 a
    1 b
    2 c
    3 d
    &gt;&gt;&gt;

Now, when I try to use it inside a `list comprehension` it gives me this error

    &gt;&gt;&gt; [i,j for i,j in enumerate(mylist)]
      File &quot;&lt;stdin&gt;&quot;, line 1
        [i,j for i,j in enumerate(mylist)]
               ^
    SyntaxError: invalid syntax

So, my question is: what is the correct way of using enumerate inside list comprehension?

Python using enumerate inside list comprehension

**Is it possible to return 2 (or more) items for each item in a list comprehension?**

What I want (example):

    [f(x), g(x) for x in range(n)]

should return `[f(0), g(0), f(1), g(1), ..., f(n-1), g(n-1)]`


So, something to replace this block of code:
    
    result = list()
    for x in range(n):
        result.add(f(x))
        result.add(g(x))

List comprehension: Returning two (or more) items for each item

&gt; **Possible Duplicate:**  
&gt; [A Transpose/Unzip Function in Python](https://stackoverflow.com/questions/19339/a-transpose-unzip-function-in-python)  

&lt;!-- End of automatically inserted text --&gt;

I have a list of tuples, where I want to unzip this list into two independent lists. I&#39;m looking for some standardized operation in Python. 

    &gt;&gt;&gt; l = [(1,2), (3,4), (8,9)]
    &gt;&gt;&gt; f_xxx (l)
    [ [1, 3, 8], [2, 4, 9] ] 

I&#39;m looking for a succinct and pythonic way to achieve this. 

Basically, I&#39;m hunting for inverse operation of [`zip()`][1] function. 


  [1]: http://docs.python.org/library/functions.html?highlight=zip#zip

How to unzip a list of tuples into individual lists?

How do you access other class variables from a list comprehension within the class definition?  The following works in Python 2 but fails in Python 3:

    class Foo:
        x = 5
        y = [x for i in range(1)]

Python 3.2 gives the error:

    NameError: global name &#39;x&#39; is not defined

Trying `Foo.x` doesn&#39;t work either.  Any ideas on how to do this in Python 3?

A slightly more complicated motivating example:

    from collections import namedtuple
    class StateDatabase:
        State = namedtuple(&#39;State&#39;, [&#39;name&#39;, &#39;capital&#39;])
        db = [State(*args) for args in [
            [&#39;Alabama&#39;, &#39;Montgomery&#39;],
            [&#39;Alaska&#39;, &#39;Juneau&#39;],
            # ...
        ]]

In this example, `apply()` would have been a decent workaround, but it is sadly removed from Python 3.

Accessing class variables from a list comprehension in the class definition

I have a list of dictionaries, and want to add a key for each element of this list.
I tried:

    result = [ item.update({&quot;elem&quot;:&quot;value&quot;}) for item in mylist ]

but the update method returns None, so my result list is full of None.

    result = [ item[&quot;elem&quot;]=&quot;value&quot; for item in mylist ]

returns a syntax error.

Content Type	Original Author	Original Content on Stackoverflow
Question	LarsVegas	View Question on Stackoverflow
Solution 1 - Python	jamylak	View Answer on Stackoverflow
Solution 2 - Python	toto_tico	View Answer on Stackoverflow
Solution 3 - Python	Mariy	View Answer on Stackoverflow

Possible to return two lists from a list comprehension?

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

However,

Solution 3 - Python

What is a regex to match a string NOT at the end of a line?

django: Purpose of django.utils.functional.SimpleLazyObject?

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