    lists = [[&#39;hello&#39;], [&#39;world&#39;, &#39;foo&#39;, &#39;bar&#39;]]
    combined = [item for sublist in lists for item in sublist]

Or:

    import itertools

    lists = [[&#39;hello&#39;], [&#39;world&#39;, &#39;foo&#39;, &#39;bar&#39;]]
    combined = list(itertools.chain.from_iterable(lists))

    from itertools import chain
    
    combined = [[&#39;hello&#39;], [&#39;world&#39;, &#39;foo&#39;, &#39;bar&#39;]]
    single = [i for i in chain.from_iterable(combined)]

I am new to opencv and trying to implement image matching between two images. For this purpose, I&#39;m trying to understand the difference between feature descriptors, descriptor extractors and descriptor matchers. I came across a lot of terms and tried to read about them on the opencv documentation website but I just can&#39;t seem to wrap my head around the concepts. I understood the basic difference here. https://stackoverflow.com/questions/6832933/difference-between-feature-detection-and-descriptor-extraction?rq=1

But I came across the following terms while studying on the topic : 
&gt;FAST, GFTT, SIFT, SURF, MSER, STAR, ORB, BRISK, FREAK, BRIEF 

I understand how FAST, SIFT, SURF work but can&#39;t seem to figure out which ones of the above are only detectors and which are extractors.

Then there are the matchers. 
&gt;FlannBased, BruteForce, knnMatch and probably some others.

After some reading, I figured that certain matchers can only be used with certain extractors as explained here. https://stackoverflow.com/questions/7232651/how-does-opencv-orb-feature-detector-work
The classification given is quite clear but it&#39;s only for a few extractors and I don&#39;t understand the difference between float and uchar. 

So basically, can someone please 

1. classify the types of detectors, extractors and matchers based on float and uchar, as mentioned, or some other type of classification?
2. explain the difference between the float and uchar classification or whichever classification is being used?
3. mention how to initialize (code) various types of detectors, extractors and matchers?

I know its asking for a lot but I&#39;ll be highly grateful.
Thank you.

Classification of detectors, extractors and matchers

I serve some of my files from my Github pages account. But when I check my &quot;js&quot; files&#39; headers, I saw following:

    Last-Modified:Sun, 10 Feb 2013 14:00:46 GMT 
    Expires:Sun, 10 Feb 2013 14:13:23 GMT
    Cache-Control:max-age=600


This file was commited 14 days ago. But I don&#39;t plan these files so frequent. So I need to make much bigger max-age value and change Last-Modified value to last commited date.

Is it possible to change headers of Github pages?

Github pages, HTTP headers

Given `lists = [[&#39;hello&#39;], [&#39;world&#39;, &#39;foo&#39;, &#39;bar&#39;]]`

How do I transform that into a single list of strings?

`combinedLists = [&#39;hello&#39;, &#39;world&#39;, &#39;foo&#39;, &#39;bar&#39;]`


Python list comprehension to join list of lists

Given <code>lists = [['hello'], ['world', 'foo', 'bar']]</code>
How do I transform that into a single list of strings?
<code>combinedLists = ['hello', 'world', 'foo', 'bar']</code>

when my function f is called with a variable I want to check if var is a pandas dataframe:

    def f(var):
        if var == pd.DataFrame():
            print &quot;do stuff&quot;

I guess the solution might be quite simple but even with 

    def f(var):
        if var.values != None:
            print &quot;do stuff&quot;

I can&#39;t get it to work like expected. 

check if variable is dataframe

I am using `url_for` to generate a redirect URL when a user has logged out:

    return redirect(url_for(&#39;.index&#39;, _external=True))

However, when I changed the page to a *https* connection, the `url_for` still gives me *http*.

I would like to explicitly ask `url_for` to add *https* at the beginning of a URL.

Can you point me how to change it? I looked at Flask docs, without luck.

Flask url_for generating http URL instead of https

I can import matplotlib but when I try to run the following:

    matplotlib.pyplot(x)

   I get:
    
    Traceback (most recent call last):
       File &quot;&lt;pyshell#31&gt;&quot;, line 1, in &lt;module&gt;
           matplotlib.pyplot(x)
    AttributeError: &#39;module&#39; object has no attribute &#39;pyplot&#39;



matplotlib has no attribute &#39;pyplot&#39;

I know I can link Flask with Apache or other web servers. But, I was thinking of running Flask as a standalone server serving multiple clients simultaneously.

Is this possible? Do I have to handle spawning multiple threads and managing them?

Can I serve multiple clients using just Flask app.run() as standalone?

I have some escaped strings that need to be unescaped. I&#39;d like to do this in Python.

For example, in Python 2.7 I can do this:


    &gt;&gt;&gt; &quot;\\123omething special&quot;.decode(&#39;string-escape&#39;)
    &#39;Something special&#39;
    &gt;&gt;&gt; 

How do I do it in Python 3?  This doesn&#39;t work:


    &gt;&gt;&gt; b&quot;\\123omething special&quot;.decode(&#39;string-escape&#39;)
    Traceback (most recent call last):
      File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
    LookupError: unknown encoding: string-escape
    &gt;&gt;&gt; 

My goal is to be able to take a string like this:

    s\000u\000p\000p\000o\000r\000t\000@\000p\000s\000i\000l\000o\000c\000.\000c\000o\000m\000

And turn it into:

    &quot;support@psiloc.com&quot;

After I do the conversion, I&#39;ll probe to see if the string I have is encoded in UTF-8 or UTF-16.




How do I .decode(&#39;string-escape&#39;) in Python 3?

Consider the following Python code with which I add in a new `list2` all the items with indices from 1 to 3 of `list1`:

    for ind, obj in enumerate(list1):
        if 4 &gt; ind &gt; 0:
            list2.append(obj)

How would you write this using list comprehension, if I have no access to the indices through enumerate?

something like:

    list2 = [x for x in list1 if 4 &gt; ind &gt; 0]

but since I have no `ind` number, would this work?

    list2 = [x for x in enumerate(list1) if 4 &gt; ind &gt; 0]

In Python list comprehension is it possible to access the item index?

We all know python&#39;s

    [f(x) for x in y if g(x)]

syntax.

However the 
[AST][1] representation of list comprehension has room for more than one &#39;if&#39; expression:

    comprehension = (expr target, expr iter, expr* ifs)

Can somebody give me an example of python code that would produce an AST with more than one &#39;if&#39; expression?

  [1]: http://docs.python.org/2/library/ast.html#abstract-grammar

python list comprehension with multiple &#39;if&#39;s

I want to compare 2 iterables and print the items which appear in both iterables.

    &gt;&gt;&gt; a = (&#39;q&#39;, &#39;r&#39;)
    &gt;&gt;&gt; b = (&#39;q&#39;)
   

    # Iterate over a. If y not in b, print y.
    # I want to see [&#39;r&#39;] printed.
    &gt;&gt;&gt; print([ y if y not in b for y in a])
                                  ^
But it gives me a invalid syntax error where the `^` has been placed.
What is wrong about this lamba function?






List comprehension with if statement

I would like to know if there is a better way to print all objects in a Python list than this :

    myList = [Person(&quot;Foo&quot;), Person(&quot;Bar&quot;)]
    print(&quot;\n&quot;.join(map(str, myList)))
    Foo
    Bar

I read this way is not really good :

    myList = [Person(&quot;Foo&quot;), Person(&quot;Bar&quot;)]
    for p in myList:
        print(p)

Isn&#39;t there something like :

    print(p) for p in myList

If not, my question is... why ? If we can do this kind of stuff with comprehensive lists, why not as a simple statement outside a list ?

Pythonic way to print list items

I have a list comprehension which approximates to:

    [f(x) for x in l if f(x)]

Where l is a list and f(x) is an expensive function which returns a list.  

I want to avoid evaluating f(x) twice for every non-empty occurance of f(x).  Is there some way to save its output within the list comprehension?

I could remove the final condition, generate the whole list and then prune it, but that seems wasteful.

__Edit__:

Two basic approaches have been suggested:

An inner generator comprehension:

    [y for y in (f(x) for x in l) if y]

or memoization.  

I think the inner generator comprehension is elegant for the problem as stated.  In actual fact I simplified the question to make it clear, I really want:

    [g(x, f(x)) for x in l if f(x)]

For this more complicated situation, I think memoization produces a cleaner end result.

Content Type	Original Author	Original Content on Stackoverflow
Question	congusbongus	View Question on Stackoverflow
Solution 1 - Python	Nicolas	View Answer on Stackoverflow
Solution 2 - Python	akira	View Answer on Stackoverflow

Python list comprehension to join list of lists

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

Github pages, HTTP headers

Classification of detectors, extractors and matchers

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