# HTTP entrypoint, handling error within a dependency?

**URL:** <https://discourse.nameko.io/t/http-entrypoint-handling-error-within-a-dependency/200>\
**Category:** googlegroup\
**Created:** [June 16, 2017, 2:37pm UTC](https://discourse.nameko.io/t/http-entrypoint-handling-error-within-a-dependency/200 "2017-06-16T14:37:50Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Richard](https://avatars.discourse-cdn.com/v4/letter/r/f0a364/32.png) [@Richard](https://discourse.nameko.io/u/Richard)\
**Post date:** [June 16, 2017, 2:37pm UTC](https://discourse.nameko.io/t/http-entrypoint-handling-error-within-a-dependency/200/1 "2017-06-16T14:37:50Z")

</div>

I have an Auth dependency which adds `self.auth` to a nameko service with  
user\_id and some other details on it.

Upon a http request, it looks at the request passed into an entrypoint,  
parses headers etc.

The issue I am having is that if there is something wrong with the headers,  
it'll raise a `ValueError` with a message.

I have a decorator on my http entrypoint which catches `ValueError` and  
returns a 400 http status code along with a message, however this doesn't  
catch the ValueError when it's coming from the dependency.

The end result being a 500 error.

How can I deal with the error from the dependency, and return status  
code/resp?

```auto
class MyService:
    auth = AuthDep()

   @handle_http_errors
   @http('GET', '/')
   def get_thing(self, request):
       return 200, 'thing'

```

Thanks

---

<div class="post-metadata">

**Author:** ![Richard](https://avatars.discourse-cdn.com/v4/letter/r/f0a364/32.png) [@Richard](https://discourse.nameko.io/u/Richard)\
**Post date:** [June 19, 2017, 9:36am UTC](https://discourse.nameko.io/t/http-entrypoint-handling-error-within-a-dependency/200/2 "2017-06-19T09:36:40Z")

</div>

I've got a hacky solution to this:

```auto
class AuthDep:

    def get_dependency(self, worker_ctx):

     try:
         do_stuff_that_sometimes_raises():
     except ValueError as exc:
         return LazyDependencyException(exc=exc)

class LazyDependencyException:
    def __init__ (self, exc):
        self.exc = exc

    def __getattr__ (self, name):
        raise self.exc

    def __call__ (self):
        raise self.exc

```

This just causes the exception to raise within the entrypoint when the  
dependency is accessed.

Is this a terrible of an idea as it seems? Is there a better way to handle  
an error within the dependency?

> **···**
>
> On Friday, June 16, 2017 at 3:37:50 PM UTC+1, Richard wrote:
> 
> > I have an Auth dependency which adds `self.auth` to a nameko service with  
> > user\_id and some other details on it.
> > 
> > Upon a http request, it looks at the request passed into an entrypoint,  
> > parses headers etc.
> > 
> > The issue I am having is that if there is something wrong with the  
> > headers, it'll raise a `ValueError` with a message.
> > 
> > I have a decorator on my http entrypoint which catches `ValueError` and  
> > returns a 400 http status code along with a message, however this doesn't  
> > catch the ValueError when it's coming from the dependency.
> > 
> > The end result being a 500 error.
> > 
> > How can I deal with the error from the dependency, and return status  
> > code/resp?
> > 
> > ```auto
> > class MyService:
> > auth = AuthDep()
> > 
> > @handle_http_errors
> > @http('GET', '/')
> > def get_thing(self, request):
> > return 200, 'thing'
> > 
> > ```
> > 
> > Thanks

---

<div class="post-metadata">

**Author:** ![David\_Szotten](https://yyz1.discourse-cdn.com/flex031/user_avatar/discourse.nameko.io/david_szotten/32/6_2.png) [@David\_Szotten](https://discourse.nameko.io/u/David_Szotten)\
**Post date:** [June 19, 2017, 10:10am UTC](https://discourse.nameko.io/t/http-entrypoint-handling-error-within-a-dependency/200/3 "2017-06-19T10:10:50Z")

</div>

possibly less hacky is to instead of `LazyDependencyException` return a  
sentinel object that your decorator could explicitly look for and bail out  
early (similar to failed auth i guess).

best,  
d

> **···**
>
> On Monday, 19 June 2017 10:36:41 UTC+1, Richard O'Dwyer wrote:
> 
> > I've got a hacky solution to this:
> > 
> > ```auto
> > class AuthDep:
> > 
> > def get_dependency(self, worker_ctx):
> > 
> > try:
> > do_stuff_that_sometimes_raises():
> > except ValueError as exc:
> > return LazyDependencyException(exc=exc)
> > 
> > class LazyDependencyException:
> > def __init__ (self, exc):
> > self.exc = exc
> > 
> > def __getattr__ (self, name):
> > raise self.exc
> > 
> > def __call__ (self):
> > raise self.exc
> > 
> > ```
> > 
> > This just causes the exception to raise within the entrypoint when the  
> > dependency is accessed.
> > 
> > Is this a terrible of an idea as it seems? Is there a better way to handle  
> > an error within the dependency?
> > 
> > On Friday, June 16, 2017 at 3:37:50 PM UTC+1, Richard wrote:
> > 
> > > I have an Auth dependency which adds `self.auth` to a nameko service with  
> > > user\_id and some other details on it.
> > > 
> > > Upon a http request, it looks at the request passed into an entrypoint,  
> > > parses headers etc.
> > > 
> > > The issue I am having is that if there is something wrong with the  
> > > headers, it'll raise a `ValueError` with a message.
> > > 
> > > I have a decorator on my http entrypoint which catches `ValueError` and  
> > > returns a 400 http status code along with a message, however this doesn't  
> > > catch the ValueError when it's coming from the dependency.
> > > 
> > > The end result being a 500 error.
> > > 
> > > How can I deal with the error from the dependency, and return status  
> > > code/resp?
> > > 
> > > ```auto
> > > class MyService:
> > > auth = AuthDep()
> > > 
> > > @handle_http_errors
> > > @http('GET', '/')
> > > def get_thing(self, request):
> > > return 200, 'thing'
> > > 
> > > ```
> > > 
> > > Thanks

---

<div class="post-metadata">

**Author:** ![mattbennett](https://yyz1.discourse-cdn.com/flex031/user_avatar/discourse.nameko.io/mattbennett/32/13_2.png) [@mattbennett](https://discourse.nameko.io/u/mattbennett)\
**Post date:** [June 19, 2017, 5:13pm UTC](https://discourse.nameko.io/t/http-entrypoint-handling-error-within-a-dependency/200/4 "2017-06-19T17:13:28Z")

</div>

Sorry for the delayed reply.

The fundamental problem here is that worker setup (and dependency  
injection) aren't handled inside the scope of a particular worker call. We  
assume that any exception outside of the service method cannot be handled  
and therefore we let it bubble out. It intentionally crashes the whole  
process. This has been discussed on the mailing list before  
\<[Redirecting to Google Groups](https://groups.google.com/forum/#!searchin/nameko-dev/worker$20setup%7Csort:relevance/nameko-dev/POBxAV71TUE/XRqyV-iHCwAJ&gt); because  
it's somewhat surprising.

Arguably worker\_setup and get\_dependency really can be attributed to a  
particular worker and errors in these methods should be returned to the  
client, rather than crashing the process. Dealing with errors in  
worker\_result and worker\_teardown are slightly more tricky, since we've  
already sent a response to the caller. But perhaps throwing a traceback and  
recovering would be better than our current design. It would be more  
in-line with what other frameworks do. It is a relatively easy change,  
although obviously not backwards compatible.

An alternative approach would be to use a "contract" between the extensions  
and explicitly ask the Auth dependency from the http entrypoint whether the  
request is valid. All bound extensions have access to the container so this  
is feasible, but it requires coupling your two extensions together, which  
isn't very nice.

@David what are your thoughts on bringing these errors into the "worker  
lifecycle"?

> **···**
>
> On Monday, June 19, 2017 at 11:10:50 AM UTC+1, David Szotten wrote:
> 
> > possibly less hacky is to instead of `LazyDependencyException` return a  
> > sentinel object that your decorator could explicitly look for and bail out  
> > early (similar to failed auth i guess).
> > 
> > best,  
> > d
> > 
> > On Monday, 19 June 2017 10:36:41 UTC+1, Richard O'Dwyer wrote:
> > 
> > > I've got a hacky solution to this:
> > > 
> > > ```auto
> > > class AuthDep:
> > > 
> > > def get_dependency(self, worker_ctx):
> > > 
> > > try:
> > > do_stuff_that_sometimes_raises():
> > > except ValueError as exc:
> > > return LazyDependencyException(exc=exc)
> > > 
> > > class LazyDependencyException:
> > > def __init__ (self, exc):
> > > self.exc = exc
> > > 
> > > def __getattr__ (self, name):
> > > raise self.exc
> > > 
> > > def __call__ (self):
> > > raise self.exc
> > > 
> > > ```
> > > 
> > > This just causes the exception to raise within the entrypoint when the  
> > > dependency is accessed.
> > > 
> > > Is this a terrible of an idea as it seems? Is there a better way to  
> > > handle an error within the dependency?
> > > 
> > > On Friday, June 16, 2017 at 3:37:50 PM UTC+1, Richard wrote:
> > > 
> > > > I have an Auth dependency which adds `self.auth` to a nameko service  
> > > > with user\_id and some other details on it.
> > > > 
> > > > Upon a http request, it looks at the request passed into an entrypoint,  
> > > > parses headers etc.
> > > > 
> > > > The issue I am having is that if there is something wrong with the  
> > > > headers, it'll raise a `ValueError` with a message.
> > > > 
> > > > I have a decorator on my http entrypoint which catches `ValueError` and  
> > > > returns a 400 http status code along with a message, however this doesn't  
> > > > catch the ValueError when it's coming from the dependency.
> > > > 
> > > > The end result being a 500 error.
> > > > 
> > > > How can I deal with the error from the dependency, and return status  
> > > > code/resp?
> > > > 
> > > > ```auto
> > > > class MyService:
> > > > auth = AuthDep()
> > > > 
> > > > @handle_http_errors
> > > > @http('GET', '/')
> > > > def get_thing(self, request):
> > > > return 200, 'thing'
> > > > 
> > > > ```
> > > > 
> > > > Thanks