    &gt;&gt;&gt; list(itertools.product(*arrays))
    [(-1, -2, -3), (-1, -2, 3), (-1, 2, -3), (-1, 2, 3), (1, -2, -3), (1, -2, 3), (1, 2, -3), (1, 2, 3)]

This will feed all the pairs as separate arguments to `product`, which will then give you the cartesian product of them.

The reason your version isn&#39;t working is that you are giving `product` only one argument. Asking for a cartesian product of one list is a trivial case, and returns a list containing only one element (the list given as argument).

    &gt;&gt;&gt; arrays = [(-1,+1), (-2,+2), (-3,+3)]
    &gt;&gt;&gt; list(itertools.product(*arrays))
    [(-1, -2, -3), (-1, -2, 3), (-1, 2, -3), (-1, 2, 3), (1, -2, -3), (1, -2, 3), (1, 2, -3), (1, 2, 3)]


you can do it in three rurch using itertools.product

    lst=[]
    arrays = [(-1,+1), (-2,+2), (-3,+3)]  
    
    import itertools 
    
    for i in itertools.product(*arrays):
             lst.append(i)

 

    print(lst)

[![enter image description here][1]][1]


  [1]: https://i.stack.imgur.com/IpRdv.png

How can I add a background image to a UILabel in an iPhone Application. I&#39;ve tried to do it through IB but with no result.

Adding Background image to UILabel

Further in my code, I check to see check if an object is null/empty.

Is there a way to set an object to null?

Is it possible to set an object to null?

  I have a list of arrays and I would like to get the cartesian product of the elements in the arrays.
  
  I will use an example to make this more concrete...
  
  
  
  
  itertools.product seems to do the trick but I am stuck in a little detail.
  
    arrays = [(-1,+1), (-2,+2), (-3,+3)];
  
  If I do
  
    cp = list(itertools.product(arrays));
  
  I get 
  
    cp = cp0 = [((-1, 1),), ((-2, 2),), ((-3, 3),)]
  
  But what I want to get is
  
    cp1 = [(-1,-2,-3), (-1,-2,+3), (-1,+2,-3), (-1,+2,+3), ..., (+1,+2,-3), (+1,+2,+3)].
  
  I have tried a few different things:
    
    cp = list(itertools.product(itertools.islice(arrays, len(arrays))));
    cp = list(itertools.product(iter(arrays, len(arrays))));
  
  They all gave me _cp0_ instead of _cp1_.
  
  Any ideas?
  
  Thanks in advance.
  
  
  

How to apply itertools.product to elements of a list of lists?

I have a list of arrays and I would like to get the cartesian product of the elements in the arrays.
I will use an example to make this more concrete...
itertools.product seems to do the trick but I am stuck in a little detail.
<pre><code class="hljs language-ini">arrays = [(-1,+1), (-2,+2), (-3,+3)];
</code></pre>
If I do
<pre><code class="hljs language-ini">cp = list(itertools.product(arrays));
</code></pre>
I get
<pre><code class="hljs language-ini">cp = cp0 = [((-1, 1),), ((-2, 2),), ((-3, 3),)]
</code></pre>
But what I want to get is
<pre><code class="hljs language-ini">cp1 = [(-1,-2,-3), (-1,-2,+3), (-1,+2,-3), (-1,+2,+3), ..., (+1,+2,-3), (+1,+2,+3)].
</code></pre>
I have tried a few different things:
<pre><code class="hljs language-ini">cp = list(itertools.product(itertools.islice(arrays, len(arrays))));
cp = list(itertools.product(iter(arrays, len(arrays))));
</code></pre>
They all gave me cp0 instead of cp1.
Any ideas?
Thanks in advance.

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 following this [tutorial][1] on a Windows 7 environment.

My settings file has this definition:

    TEMPLATE_DIRS = (
        &#39;C:/django-project/myapp/mytemplates/admin&#39;
    )

I got the `base_template` from  the template `admin/base_site.html` from within the default Django admin template directory in the source code of Django itself (django/contrib/admin/templates) into an admin subdirectory of `myapp` directory as the tutorial instructed, but it doesn&#39;t seem to take affect for some reason.

Any clue of what might be the problem?


  [1]: http://docs.djangoproject.com/en/dev/intro/tutorial02/#intro-tutorial02

What is the path that Django uses for locating and loading templates?

I just started using SQLAlchemy and get a DetachedInstanceError and can&#39;t find much information on this anywhere. I am using the instance outside a session, so it is natural that SQLAlchemy is unable to load any relations if they are not already loaded, however, the attribute I am accessing is not a relation, in fact this object has no relations at all. I found solutions such as eager loading, but I can&#39;t apply to this because this is not a relation. I even tried &quot;touching&quot; this attribute before closing the session, but it still doesn&#39;t prevent the exception. What could be causing this exception for a non-relational property even after it has been successfully accessed once before? Any help in debugging this issue is appreciated. I will meanwhile try to get a reproducible stand-alone scenario and update here.

Update: This is the actual exception message with a few stacks:

      File &quot;/home/hari/bin/lib/python2.6/site-packages/SQLAlchemy-0.6.1-py2.6.egg/sqlalchemy/orm/attributes.py&quot;, line 159, in __get__
        return self.impl.get(instance_state(instance), instance_dict(instance))
      File &quot;/home/hari/bin/lib/python2.6/site-packages/SQLAlchemy-0.6.1-py2.6.egg/sqlalchemy/orm/attributes.py&quot;, line 377, in get
        value = callable_(passive=passive)
      File &quot;/home/hari/bin/lib/python2.6/site-packages/SQLAlchemy-0.6.1-py2.6.egg/sqlalchemy/orm/state.py&quot;, line 280, in __call__
        self.manager.deferred_scalar_loader(self, toload)
      File &quot;/home/hari/bin/lib/python2.6/site-packages/SQLAlchemy-0.6.1-py2.6.egg/sqlalchemy/orm/mapper.py&quot;, line 2323, in _load_scalar_attributes
        (state_str(state)))
    DetachedInstanceError: Instance &lt;ReportingJob at 0xa41cd8c&gt; is not bound to a Session; attribute refresh operation cannot proceed

The partial model looks like this:

    metadata = MetaData()
    ModelBase = declarative_base(metadata=metadata)
    
    class ReportingJob(ModelBase):
        __tablename__ = &#39;reporting_job&#39;
    
        job_id         = Column(BigInteger, Sequence(&#39;job_id_sequence&#39;), primary_key=True)
        client_id      = Column(BigInteger, nullable=True)

And the field client_id is what is causing this exception with a usage like the below:

Query:

        jobs = session \
                .query(ReportingJob) \
                .filter(ReportingJob.job_id == job_id) \
                .all()
        if jobs:
            # FIXME(Hari): Workaround for the attribute getting lazy-loaded.
            jobs[0].client_id
            return jobs[0]

This is what triggers the exception later out of the session scope:

            msg = msg + &quot;, client_id: %s&quot; % job.client_id


SQLAlchemy DetachedInstanceError with regular attribute (not a relation)

I have a list of regexes in python, and a string. Is there an elegant way to check if the at least one regex in the list matches the string? By elegant, I mean something better than simply looping through all of the regexes and checking them against the string and stopping if a match is found.

Basically, I had this code:

    list = [&#39;something&#39;,&#39;another&#39;,&#39;thing&#39;,&#39;hello&#39;]
    string = &#39;hi&#39;
    if string in list:
      pass # do something
    else:
      pass # do something else

Now I would like to have some regular expressions in the list, rather than just strings, and I am wondering if there is an elegant solution to check for a match to replace `if string in list:`.

Thanks in advance.

Python: Elegant way to check if at least one regex in list matches a string

Is there any short way to achieve what the APT (*Advanced Package Tool*) command line interface does in Python?

I mean, when the package manager prompts a yes/no question followed by `[Yes/no]`, the script accepts `YES/Y/yes/y` or &lt;kbd&gt;Enter&lt;/kbd&gt; (defaults to `Yes` as hinted by the capital letter).

The only thing I find in the official docs is `input` and `raw_input`...

I know it&#39;s not that hard to emulate, but it&#39;s annoying to rewrite :|

APT command line interface-like yes/no input?

When is it better to use [`zip`](https://docs.python.org/2/library/functions.html#zip) instead of [`itertools.izip`](https://docs.python.org/2/library/itertools.html#itertools.izip)?

When is it better to use zip instead of izip?

itertools.permutations generates where its elements are treated as unique based on their position, not on their value. So basically I want to avoid duplicates like this:

    &gt;&gt;&gt; list(itertools.permutations([1, 1, 1]))
    [(1, 1, 1), (1, 1, 1), (1, 1, 1), (1, 1, 1), (1, 1, 1), (1, 1, 1)]

Filtering afterwards is not possible because the amount of permutations is too large in my case.

Does anybody know of a suitable algorithm for this?

Thank you very much!

**EDIT:**

What I basically want is the following:

    x = itertools.product((0, 1, &#39;x&#39;), repeat=X)
    x = sorted(x, key=functools.partial(count_elements, elem=&#39;x&#39;))

which is not possible because ``sorted`` creates a list and the output of itertools.product is too large.

Sorry, I should have described the actual problem.

permutations with unique values

I am writing a code to take an enormous textfile (several GB) N lines at a time, process that batch, and move onto the next N lines until I have completed the entire file.  (I don&#39;t care if the last batch isn&#39;t the perfect size).

I have been reading about using itertools islice for this operation.  I think I am halfway there:


    from itertools import islice
    N = 16
    infile = open(&quot;my_very_large_text_file&quot;, &quot;r&quot;)
    lines_gen = islice(infile, N)
    
    for lines in lines_gen:
         ...process my lines...

The trouble is that I would like to process the next batch of 16 lines, but I am missing something

Python how to read N number of lines at a time

I’m having trouble wrapping my head around a algorithm I’m try to implement. I have two lists and want to take particular combinations from the two lists.

Here’s an example.
```
names = [&#39;a&#39;, &#39;b&#39;]
numbers = [1, 2]
```
the output in this case would be:
```
[(&#39;a&#39;, 1), (&#39;b&#39;, 2)]
[(&#39;b&#39;, 1), (&#39;a&#39;, 2)]
```
I might have more names than numbers, i.e. `len(names) &gt;= len(numbers)`. Here&#39;s an example with 3 names and 2 numbers:
```
names = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;]
numbers = [1, 2]
```
output:
```
[(&#39;a&#39;, 1), (&#39;b&#39;, 2)]
[(&#39;b&#39;, 1), (&#39;a&#39;, 2)]
[(&#39;a&#39;, 1), (&#39;c&#39;, 2)]
[(&#39;c&#39;, 1), (&#39;a&#39;, 2)]
[(&#39;b&#39;, 1), (&#39;c&#39;, 2)]
[(&#39;c&#39;, 1), (&#39;b&#39;, 2)]
```

Permutations between two lists of unequal length

I&#39;m working with Python itertools and using groupby to sort a bunch of pairs by the last element. I&#39;ve gotten it to sort and I can iterate through the groups just fine, but I would really love to be able to get the length of each group without having to iterate through each one, incrementing a counter.

The project is cluster some data points. I&#39;m working with pairs of (numpy.array, int) where the numpy array is a data point and the integer is a cluster label

Here&#39;s my relevant code:

    data = sorted(data, key=lambda (point, cluster):cluster)
    for cluster,clusterList in itertools.groupby(data, key=lambda (point, cluster):cluster):
        if len(clusterList) &lt; minLen:

On the last line: `if len(clusterList) &lt; minLen:`, I get an error that 
&gt;object of type &#39;itertools._grouper&#39; has no len()

I&#39;ve looked up the operations available for `_groupers`, but can&#39;t find anything that seems to provide the length of a group.


Python: How to get the length of itertools _grouper

Given 2 arrays `Array1 = {a,b,c...n}` and `Array2 = {10,20,15....x}` how can I generate all possible combination as Strings **a(i) b(j) c(k) n(p)** 
where 

    1 &lt;= i &lt;= 10,  1 &lt;= j &lt;= 20 , 1 &lt;= k &lt;= 15,  .... 1 &lt;= p &lt;= x

Such as:

    a1 b1 c1 .... n1  
    a1 b1 c1..... n2  
    ......  
    ......  
    a10 b20 c15 nx (last combination)


So in all total number of combination = product of elements of `array2 = 
(10 X 20 X 15 X ..X x)`

Similar to a Cartesian product, in which the second array defines the upper limit for each element in first array.

Example with fixed numbers,  
    
        Array x =  [a,b,c]
        Array y =  [3,2,4] 

So we will have 3*2*4 = 24 combinations. Results should be:
        
        a1 b1 c1  
        a1 b1 c2  
        a1 b1 c3  
        a1 b1 c4  
        
        a1 b2 c1  
        a1 b2 c2  
        a1 b2 c3  
        a1 b2 c4
          
        
        a2 b1 c1  
        a2 b1 c2  
        a2 b1 c3  
        a2 b1 c4  
        
        a2 b2 c1  
        a2 b2 c2  
        a2 b2 c3  
        a2 b2 c4
          
        
        a3 b1 c1  
        a3 b1 c2  
        a3 b1 c3  
        a3 b1 c4  
        
        a3 b2 c1  
        a3 b2 c2  
        a3 b2 c3  
        a3 b2 c4 (last)

Generating all Possible Combinations

I have a class structure like so:

    Person
    Dogs (dog 1, dog 2, etc)
    Puppies (puppy A, puppy B, etc)

There is one person. He has 1..n dogs. Each dog has 1..n puppies.

I want a list of all the possible combination of puppies, taking 1 puppy from each dog. Eg:

dog 1 puppy A, dog 2 puppy A
dog 1 puppy A, dog 2 puppy B
dog 1 puppy B, dog 2 puppy A
dog 1 puppy B, dog 2 puppy B

If it was in sql tables, i&#39;d do something like the following to &#39;multiply&#39; the tables:

    select * from puppies a, puppies b where a.parent=&#39;dog1&#39; and b.parent=&#39;dog2&#39;

Is there some linq-ish way to do this kinda thing???

Thanks so much

Is there a good LINQ way to do a cartesian product?

I wish to produce the Cartesian product of 2 lists in Haskell, but I cannot work out how to do it. The cartesian product gives all combinations of the list elements:

    xs = [1,2,3]
    ys = [4,5,6]

    cartProd :: [a] -&gt; [b] -&gt; [(a,b)]
    cartProd xs ys ==&gt; [(1,4),(1,5),(1,6),(2,4),(2,5),(2,6),(3,4),(3,5),(3,6)]

This is not an actual homework question and is not related to any such question, but the way in which this problem is solved may help with one I am stuck on.



Cartesian product of 2 lists in Haskell

Say that I have an array like the following:

    Array
    (
        [arm] =&gt; Array
            (
                [0] =&gt; A
                [1] =&gt; B
                [2] =&gt; C
            )
        [gender] =&gt; Array
            (
                [0] =&gt; Female
                [1] =&gt; Male
            )
        [location] =&gt; Array
            (
                [0] =&gt; Vancouver
                [1] =&gt; Calgary
            )
    )

How can I find the cartesian product while preserving the keys of the outer associative array and using them in the inner ones? The result of the algorithm should be this:

    Array
    (
        [0] =&gt; Array
            (
                [arm] =&gt; A
                [gender] =&gt; Female
                [location] =&gt; Vancouver
            )
    
        [1] =&gt; Array
            (
                [arm] =&gt; A
                [gender] =&gt; Female
                [location] =&gt; Calgary
            )
    
        [2] =&gt; Array
            (
                [arm] =&gt; A
                [gender] =&gt; Male
                [location] =&gt; Vancouver
            )
    
    ...etc.

I&#39;ve looked up quite a number of cartesian product algorithms but I&#39;m getting stuck on the specifics of how to preserve the associative keys. The current algorithm I am using gives numerical indices only:

        $result = array();
        foreach ($map as $a) {
            if (empty($result)) {
                $result = $a;
                continue;
            }
            $res = array();
            foreach ($result as $r) {
                foreach ($a as $v) {
                    $res[] = array_merge((array)$r, (array)$v);
                }
            }
            $result = $res;
        }
        
        print_r($result);

Any help would be appreciated.

Finding cartesian product with PHP associative arrays

I have two numpy arrays that define the x and y axes of a grid.  For example:

    x = numpy.array([1,2,3])
    y = numpy.array([4,5])

I&#39;d like to generate the Cartesian product of these arrays to generate:

    array([[1,4],[2,4],[3,4],[1,5],[2,5],[3,5]])

In a way that&#39;s not terribly inefficient since I need to do this many times in a loop.  I&#39;m assuming that converting them to a Python list and using `itertools.product` and back to a numpy array is not the most efficient form.

Content Type	Original Author	Original Content on Stackoverflow
Question	W7GVR	View Question on Stackoverflow
Solution 1 - Python	interjay	View Answer on Stackoverflow
Solution 2 - Python	rkhayrov	View Answer on Stackoverflow
Solution 3 - Python	Wojciech Moszczyński	View Answer on Stackoverflow

How to apply itertools.product to elements of a list of lists?

Python Problem Overview

Python Solutions

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Solution 3 - Python

Is it possible to set an object to null?

Adding Background image to UILabel

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