How to check if a value is in a form POST

Question

I want to query if an input value is in a form post and raise an error if it is not:

    try:
        next_page = request.POST.get('next_page')
    except AttributeError:
        raise ImproperlyConfigured(
            "No URL to redirect to.  Either provide a url or define"
            " a get_absolute_url method on the Model.")

However, if next_page is not provided this code results in next_page = None and the error is not raised. Where am I going wrong?

`try` is not always better than `if`, in fact in this case I'd argue that `if ... else ...` would be more readable. Your example fails because `get()` on a dictionary always returns something (`None` by default). You could have done `next_page = request.POST['next_page']` and caught `KeyError` exception. — dirkgroten, Jan 07 '20 at 21:45
No, `request.POST.get('next_page')` will never throw any error, but `request.POST['next_page']` will throw a `KeyError` if the key isn't present. — dirkgroten, Jan 07 '20 at 21:56
And use bracket-deference rather than `get`, if you want to go that way. `get` doesn't throw anything on a missing key. — Peter DeGlopper, Jan 07 '20 at 21:56
However, this doesn't account for the possibility that `next_page` is present but empty. That's why I would keep to a simple `if ... else ...`: `if next_page: redirect() else: raise ...`. This covers the case that `next_page` is `None` but also `''` — dirkgroten, Jan 07 '20 at 21:57

score 2 · Accepted Answer · answered Jan 07 '20 at 22:50

2

To check if a key is in the POST dictionary you can use a regular in

if 'next_page' not in request.POST:
    raise ImproperlyConfigured(
        "No URL to redirect to.  Either provide a url or define"
        " a get_absolute_url method on the Model.")

answered Jan 07 '20 at 22:50

Iain Shelvington

31,030
3
31
50

ekneiling · Answer 2 · 2020-01-07T22:16:03.993

1

Rather than use a try except block, you could just use a normal conditional clause to check if next_page is not None. (For clarity, I added a default value of None to the get method call if the 'next_page' key isn't found)

next_page = request.POST.get('next_page', None)

if next_page is not None:
   # redirect code
else:
    raise ImproperlyConfigured(
            "No URL to redirect to.  Either provide a url or define"
            " a get_absolute_url method on the Model.")

EDIT:

Another option based on asking forgiveness rather than permission:

try:
    redirect(request.POST.get('next_page'))
except KeyError:
    raise ImproperlyConfigured("text")

Here you don't need to check if the next_page value is None, you rely on the redirect function throwing an error if it receives None. You then catch that, and raise your Exception.

edited Jan 07 '20 at 22:16

answered Jan 07 '20 at 21:37

ekneiling

159
1
8

Thanks, that's helpful. Why doesn't my example work though? What's failing? I understood it's always better to `try` than `if` isn't it? – alias51 Jan 07 '20 at 21:41
1

get() will return None if the key doesn’t exist by default, passing None as the second argument is redundant – Iain Shelvington Jan 07 '20 at 21:42
@IainShelvington Thanks. Should I be using `if` in this case? I understood it's always better / lower overhead to use `try` and `except`? – alias51 Jan 07 '20 at 21:42
@alias51`try except` and `if` statements aren't interchangeable where you should favor one over the other for performance. `try except` is primarily for error handling and `if` is for logical flow. Use whatever makes the most sense in the situation. In your case, you're determining `if` a value is `None` or not, so using an `if` statement is perfectly fine. Also, the `except` clause catches exceptions, not `None` values, which is why your example wasn't working as you expected. – ekneiling Jan 07 '20 at 22:04
2

https://stackoverflow.com/questions/12265451/ask-forgiveness-not-permission-explain has some discussion of the general philosophy of exceptions (forgiveness) vs. checking first (permission). – Peter DeGlopper Jan 07 '20 at 22:07
You need to use `request.POST['next_page']` to get a `KeyError` – Iain Shelvington Jan 07 '20 at 22:47

How to check if a value is in a form POST

2 Answers2