According to [PEP-0526](https://www.python.org/dev/peps/pep-0526/#global-and-local-variable-annotations), you should annotate the types first, then do the unpacking

    a: int
    b: int
    a, b = t

I&#39;m facing this weird issue in NodeJS when using with Passport.js, Express and Mongoose. Basically, I get an error saying &quot;Cannot set headers after they are sent to the client&quot; even though I don&#39;t send more than one header.

I&#39;ve read other posts and tried them out as well, and none of them worked.

 - 
https://stackoverflow.com/questions/43055600/app-get-is-there-any-difference-between-res-send-vs-return-res-send
 - https://stackoverflow.com/questions/49740710/error-err-http-headers-sent-cannot-set-headers-after-they-are-sent-to-the-cli
 - https://stackoverflow.com/questions/47753743/cannot-set-headers-after-they-are-sent-to-the-client

I&#39;ve dug through github issues and I can&#39;t seem to find a solution. I get the problem that this error is triggered when I send multiple response headers, but the fact is that I am not sending multiple headers. It seems just weird.

This is my stack trace:

    

&gt; (node:9236) DeprecationWarning: current URL string parser is deprecated, and will be removed in a future version. To use the new parser, pass option { useNewUrlParser: true } to MongoClient.connect.

&gt; Server Running on port 5000  
&gt; MongoDB Connected Error  
&gt;[ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the
&gt; client  
&gt; &amp;nbsp;&amp;nbsp;at validateHeader (_http_outgoing.js:503:11)  
&gt; &amp;nbsp;&amp;nbsp; at ServerResponse.setHeader (_http_outgoing.js:510:3)  
&gt; &amp;nbsp;&amp;nbsp; at ServerResponse.header (/Users/lourdesroashan/code/github/devlog/node_modules/express/lib/response.js:767:10)  
&gt; &amp;nbsp;&amp;nbsp; at ServerResponse.json (/Users/lourdesroashan/code/github/devlog/node_modules/express/lib/response.js:264:10)  
&gt; &amp;nbsp;&amp;nbsp; at Profile.findOne.then.profile (/Users/lourdesroashan/code/github/devlog/routes/api/profile.js:27:30)  
&gt; &amp;nbsp;&amp;nbsp; at &amp;lt;anonymous&amp;gt;

This is my server code:

    router.get(&quot;/userprofile&quot;, passport.authenticate(&#39;jwt&#39;, { session: false }), (req, res) =&gt; {
    
      Profile.findOne({ user: req.user.id }).then(profile =&gt; {
        if (!profile) {
          return res.status(404).json({ error: &quot;No Profile Found&quot; });
        }
        else {
          res.json(profile);
        }
      }).catch(err =&gt; {
        console.log(err);
      })
    });

I understand what the error means, but from what I know, I don&#39;t think I am sending multiple headers, I even checked by console.log that only one of the blocks is run.

Thank you so much in advance! :)

Full Code at: https://github.com/lourdesr/devlog


**EDIT:**

I figured it out. It was a problem in my passport.js while trying to get the authenticated user. I forgot to use &#39;return&#39; on the &#39;done&#39; method, which had caused it. Just added the return statement and it worked!

ERR_HTTP_HEADERS_SENT: Cannot set headers after they are sent to the client

I am using [Navigation][1] from [Android Jetpack][2] to navigate between  screens.
Now I want to set startDestination dynamically. 

I have an Activity named MainActivity
And two Fragments, FragmentA &amp; FragmentB.

    var isAllSetUp : Boolean = // It is dynamic and I’m getting this from Preferences.
    
        If(isAllSetUp)
        {
         // show FragmentA
        }
        else
        {
         //show FragmentB
        }

I want to set above flow using Navigation Architecture Component. Currently I have used startDestionation as below but it’s not fulfilling my requirement.

    &lt;navigation xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
       xmlns:app=&quot;http://schemas.android.com/apk/res-auto&quot;
       xmlns:tools=&quot;http://schemas.android.com/tools&quot;
       android:id=&quot;@+id/lrf_navigation&quot;
       app:startDestination=&quot;@id/fragmentA&quot;&gt;
    
       &lt;fragment
           android:id=&quot;@+id/fragmentA&quot;
           android:name=&quot;com.mindinventory.FragmentA&quot;
           android:label=&quot;fragment_a&quot;
           tools:layout=&quot;@layout/fragment_a&quot; /&gt;
    &lt;/navigation&gt;



Is it possible to set startDestination conditionally using Android Navigation Architecture Component?


  [1]: https://developer.android.com/topic/libraries/architecture/navigation/navigation-implementing
  [2]: https://developer.android.com/jetpack/

Is it possible to set startDestination conditionally using Android Navigation Architecture Component(Android Jetpack)?

Is it possible to use type hinting when unpacking a tuple?  I want to do this, but it results in a `SyntaxError`:

    from typing import Tuple
    
    t: Tuple[int, int] = (1, 2)
    a: int, b: int = t
    #     ^ SyntaxError: invalid syntax

Type hints when unpacking a tuple?

Is it possible to use type hinting when unpacking a tuple? I want to do this, but it results in a <code>SyntaxError</code>:
<pre><code class="hljs language-python">from typing import Tuple

t: Tuple[int, int] = (1, 2)
a: int, b: int = t
# ^ SyntaxError: invalid syntax
</code></pre>

Pandas has both `isna()` and `isnull()`. I usually use `isnull()` to detect missing values and have never met the case so that I had to use other than that.
So, when to use `isna()`?

Pandas isna() and isnull(), what is the difference?

The problem originates when I start by cloning a git project that uses pipenv, so it has a Pipfile + Pipfile.lock. I want to use a virtual environment with the project so I run `pipenv shell`. I now have a virtual environment created and I am inside the virtual environment. The project obviously has a lot of dependencies (listed in the Pipfile). I don&#39;t want to have to go through the list in the Pipfile one by one and install them using `pipenv install &lt;package_name&gt;`. Is there a pipenv/pip command that installs all the packages from a Pipfile I already have? Or maybe I need to set up the environment differently than running `pipenv shell`?

How to install dependencies from a copied pipfile inside a virtual environment?

I have a pandas series with boolean entries.  I would like to get a list of indices where the values are `True`.

For example the input `pd.Series([True, False, True, True, False, False, False, True])`

should yield the output `[0,2,3,7]`.

I can do it with a list comprehension, but is there something cleaner or faster?

Getting a list of indices where pandas boolean series is True

I have a method inside a class that needs to do a lot of work in a loop, and I would like to spread the work over all of my cores.

I wrote the following code, which works if I use normal `map()`, but with `pool.map()` returns an error.

    import multiprocessing
    pool = multiprocessing.Pool(multiprocessing.cpu_count() - 1)
    
    class OtherClass:
      def run(sentence, graph):
        return False
    
    class SomeClass:
      def __init__(self):
        self.sentences = [[&quot;Some string&quot;]]
        self.graphs = [&quot;string&quot;]
    
      def some_method(self):
          other = OtherClass()
    
          def single(params):
              sentences, graph = params
              return [other.run(sentence, graph) for sentence in sentences]
    
          return list(pool.map(single, zip(self.sentences, self.graphs)))
    
    
    SomeClass().some_method()

### Error 1:

&gt; AttributeError: Can&#39;t pickle local object &#39;SomeClass.some_method.&lt;locals&gt;.single&#39;


Why can&#39;t it pickle `single()`? I even tried to move `single()` to the global module scope (not inside the class - makes it independent of the context):

    import multiprocessing
    pool = multiprocessing.Pool(multiprocessing.cpu_count() - 1)
    
    class OtherClass:
      def run(sentence, graph):
        return False
    
    
    def single(params):
        other = OtherClass()
        sentences, graph = params
        return [other.run(sentence, graph) for sentence in sentences]
    
    class SomeClass:
      def __init__(self):
        self.sentences = [[&quot;Some string&quot;]]
        self.graphs = [&quot;string&quot;]
    
      def some_method(self):
          return list(pool.map(single, zip(self.sentences, self.graphs)))
    
    
    SomeClass().some_method()

and I get the following ...
### Error 2:

&gt; AttributeError: Can&#39;t get attribute &#39;single&#39; on module &#39;__main__&#39; from &#39;.../test.py&#39;



Python multiprocessing.Pool: AttributeError

I have a Windows 10 PC and I want to install pyaudio to use it with my chatbot, powered by chatterbot.

I tried 2 different ways to install pyaudio.

The first way is doing this on the command prompt:

    python -m pip install PyAudio

This is the result:

       C:\Users\Waaberi&gt;python -m pip install PyAudio
    Collecting PyAudio
      Using cached https://files.pythonhosted.org/packages/ab/42/b4f04721c5c5bfc196ce156b3c768998ef8c0ae3654ed29ea5020c749a6b/PyAudio-0.2.11.tar.gz
    Installing collected packages: PyAudio
      Running setup.py install for PyAudio ... error
        Complete output from command C:\Users\Waaberi\AppData\Local\Programs\Python\Python37-32\python.exe -u -c &quot;import setuptools, tokenize;__file__=&#39;C:\\Users\\Waaberi\\AppData\\Local\\Temp\\pip-install-e5le61j0\\PyAudio\\setup.py&#39;;f=getattr(tokenize, &#39;open&#39;, open)(__file__);code=f.read().replace(&#39;\r\n&#39;, &#39;\n&#39;);f.close();exec(compile(code, __file__, &#39;exec&#39;))&quot; install --record C:\Users\Waaberi\AppData\Local\Temp\pip-record-adj3zivl\install-record.txt --single-version-externally-managed --compile:
        running install
        running build
        running build_py
        creating build
        creating build\lib.win32-3.7
        copying src\pyaudio.py -&gt; build\lib.win32-3.7
        running build_ext
        building &#39;_portaudio&#39; extension
        error: Microsoft Visual C++ 14.0 is required. Get it with &quot;Microsoft Visual C++ Build Tools&quot;: http://landinghub.visualstudio.com/visual-cpp-build-tools
    
        ----------------------------------------
    Command &quot;C:\Users\Waaberi\AppData\Local\Programs\Python\Python37-32\python.exe -u -c &quot;import setuptools, tokenize;__file__=&#39;C:\\Users\\Waaberi\\AppData\\Local\\Temp\\pip-install-e5le61j0\\PyAudio\\setup.py&#39;;f=getattr(tokenize, &#39;open&#39;, open)(__file__);code=f.read().replace(&#39;\r\n&#39;, &#39;\n&#39;);f.close();exec(compile(code, __file__, &#39;exec&#39;))&quot; install --record C:\Users\Waaberi\AppData\Local\Temp\pip-record-adj3zivl\install-record.txt --single-version-externally-managed --compile&quot; failed with error code 1 in C:\Users\Waaberi\AppData\Local\Temp\pip-install-e5le61j0\PyAudio\

The second way is entering this command:

    python pip install python-pyaudio

and it does nothing.




I can&#39;t install pyaudio on Windows? How to solve &quot;error: Microsoft Visual C++ 14.0 is required.&quot;?

I am trying out Seaborn to make my plot visually better than matplotlib. I have a dataset which has a column &#39;Year&#39; which I want to plot on the X-axis and 4 Columns say A,B,C,D on the Y-axis using different coloured lines. I was trying to do this using the sns.lineplot method but it allows for only one variable on the X-axis and one on the Y-axis. I tried doing this

    sns.lineplot(data_preproc[&#39;Year&#39;],data_preproc[&#39;A&#39;], err_style=None)
    sns.lineplot(data_preproc[&#39;Year&#39;],data_preproc[&#39;B&#39;], err_style=None)
    sns.lineplot(data_preproc[&#39;Year&#39;],data_preproc[&#39;C&#39;], err_style=None)
    sns.lineplot(data_preproc[&#39;Year&#39;],data_preproc[&#39;D&#39;], err_style=None)
But this way I don&#39;t get a legend in the plot to show which coloured line corresponds to what. I tried checking the documentation but couldn&#39;t find a proper way to do this.

How do I create a multiline plot using seaborn?

Say a I have a dataclass in python3. I want to be able to hash and order these objects. I do not want these to be immutable. 

I only want them ordered/hashed on id. 

I see in the docs that I can just implement \__hash__ and all that but I&#39;d like to get datacalsses to do the work for me because they are intended to handle this.

    from dataclasses import dataclass, field
    
    @dataclass(eq=True, order=True)
    class Category:
        id: str = field(compare=True)
        name: str = field(default=&quot;set this in post_init&quot;, compare=False)
    
    a = sorted(list(set([ Category(id=&#39;x&#39;), Category(id=&#39;y&#39;)])))

    Traceback (most recent call last):
      File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
    TypeError: unhashable type: &#39;Category&#39;



How can I make a python dataclass hashable without making them immutable?

I&#39;m working on a python3 project where we use the `typing` module type hints throughout.


It seems that we use `typing.Dict` and `typing.Mapping` pretty much interchangeably.

Is there a reason to prefer one over the other?

Python type hints: typing.Mapping vs. typing.Dict

There&#39;s a function which takes optional arguments.

    def alpha(p1=&quot;foo&quot;, p2=&quot;bar&quot;):
         print(&#39;{0},{1}&#39;.format(p1, p2))

Let me iterate over what happens when we use that function in different ways:

    &gt;&gt;&gt; alpha()
    foo,bar
    &gt;&gt;&gt; alpha(&quot;FOO&quot;)
    FOO,bar
    &gt;&gt;&gt; alpha(p2=&quot;BAR&quot;)
    foo,BAR
    &gt;&gt;&gt; alpha(p1=&quot;FOO&quot;, p2=None)
    FOO,None
Now consider the case where I want to call it like `alpha(&quot;FOO&quot;, myp2)` and `myp2` will either contain a value to be passed, or be `None`. But even though the function handles `p2=None`, I want it to use its default value `&quot;bar&quot;` instead.  
Maybe that&#39;s worded confusingly, so let me reword that:  

&gt; If `myp2 is None`, call `alpha(&quot;FOO&quot;)`.   Else, call `alpha(&quot;FOO&quot;, myp2)`.

The distinction is relevant because `alpha(&quot;FOO&quot;, None)` has a different result than `alpha(&quot;FOO&quot;)`.  

**How can I concisely (but readably) make this distinction?**  

One possibility would usually be to [check for None within `alpha`](https://stackoverflow.com/questions/16576553/python-only-pass-arguments-if-the-variable-exists), which would be encouraged because that would make the code safer. But assume that `alpha` is used in other places where it is actually supposed to handle `None` as it does.  

**I&#39;d like to handle that on the caller-side**.  

One possibility is to do a case distinction:

    if myp2 is None:
        alpha(&quot;FOO&quot;)
    else:
        alpha(&quot;FOO&quot;, myp2)
But that can quickly become much code when there are multiple such arguments. (exponentially, 2^n)

Another possibility is to simply do `alpha(&quot;FOO&quot;, myp2 or &quot;bar&quot;)`, but _that requires us to know the default value_. Usually, I&#39;d probably go with this approach, but I might later change the default values for `alpha` and this call would then need to be updated manually in order to still call it with the (new) default value.

I am using python 3.4 but it would be best if your answers can provide a good way that works in any python version.


----------
The question is technically finished here, but I reword some requirement again, since the first answer did gloss over that:  
I want the behaviour of `alpha` with its default values `&quot;foo&quot;, &quot;bar&quot;` preserved in general, so it is (probably) not an option to change `alpha` itself.  
In yet again other words, assume that `alpha` is being used somewhere else as `alpha(&quot;FOO&quot;, None)` where the output `FOO,None` is expected behaviour.

Call function without optional arguments if they are None

I have this piece of code:

    
    import enum
    
    
    class Color(enum.Enum):
        RED = &#39;1&#39;
        BLUE = &#39;2&#39;
        GREEN = &#39;3&#39;
    
    
    def get_color_return_something(some_color):
        pass
    
How do I properly add type annotations to the `some_color` variable in this function, if I suppose that I&#39;ll receive an enum attribute from the Color enum (for example: `Color.RED`)?

Type annotations for Enum attribute

I have a function that takes a tuple of different lengths as an argument:

```
from typing import Tuple


def process_tuple(t: Tuple[str]):
    # Do nasty tuple stuff

process_tuple((&quot;a&quot;,))
process_tuple((&quot;a&quot;, &quot;b&quot;))
process_tuple((&quot;a&quot;, &quot;b&quot;, &quot;c&quot;))

```

When I annotate function like mentioned above, I get these error messages

```
fool.py:9: error: Argument 1 to &quot;process_tuple&quot; has incompatible type &quot;Tuple[str, str]&quot;; expected &quot;Tuple[str]&quot;
fool.py:10: error: Argument 1 to &quot;process_tuple&quot; has incompatible type &quot;Tuple[str, str, str]&quot;; expected &quot;Tuple[str]&quot;
```

`process_tuple` really works with tuples and I use them as immutable lists of variable length. I haven&#39;t found any consensus on this topic on the internet, so I wonder how should I annotate this kind of input. 

How to annotate function that takes a tuple of variable length? (variadic tuple type annotation)

[`TypeVar`](https://docs.python.org/3/library/typing.html#typing.TypeVar) and [`NewType`](https://docs.python.org/3/library/typing.html#newtype) seem related but I&#39;m not sure when I&#39;m supposed to use each or what the difference is at runtime and statically.

What is the difference between TypeVar and NewType?

I have updated my class definitions to make use of the newly introduced property type hints, like this:

```php
class Foo {
    
    private int $id;
    private ?string $val;
    private DateTimeInterface $createdAt;
    private ?DateTimeInterface $updatedAt;
    
    public function __construct(int $id) {
        $this-&gt;id = $id;
    }


    public function getId(): int { return $this-&gt;id; }
    public function getVal(): ?string { return $this-&gt;val; }
    public function getCreatedAt(): ?DateTimeInterface { return $this-&gt;createdAt; }
    public function getUpdatedAt(): ?DateTimeInterface { return $this-&gt;updatedAt; }
    
    public function setVal(?string $val) { $this-&gt;val = $val; }
    public function setCreatedAt(DateTimeInterface $date) { $this-&gt;createdAt = $date; }
    public function setUpdatedAt(DateTimeInterface $date) { $this-&gt;updatedAt = $date; }
}
```

But when trying to save my entity on Doctrine I am getting an error saying:

&gt; Typed property must not be accessed before initialization 

This not only happens with `$id` or `$createdAt`, but also happen with `$value` or `$updatedAt`, which are nullable properties.

Why I am suddenly getting a &quot;Typed property must not be accessed before initialization&quot; error when introducing properties type hints?

I&#39;m really unsure why this isn&#39;t working. Here is the important part of the code (it&#39;s from a leetcode challenge).
The first line throws the NameError.

```python
def totalFruit(self, tree: List[int]) -&gt; int:
    pass
```

If I try importing `List` first I get an error `No module named &#39;List&#39;`. I&#39;m using Python 3.7.3 from Anaconda.

Content Type	Original Author	Original Content on Stackoverflow
Question	Cai	View Question on Stackoverflow
Solution 1 - Python	Patrick Haugh	View Answer on Stackoverflow

Type hints when unpacking a tuple?

Python Problem Overview

Python Solutions

Solution 1 - Python

Is it possible to set startDestination conditionally using Android Navigation Architecture Component(Android Jetpack)?

ERR_HTTP_HEADERS_SENT: Cannot set headers after they are sent to the client

Attributions