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comp.lang.python

Exception or not

Monica Leko

3/3/2008 3:33:00 PM

Suppose you have some HTML forms which you would like to validate.
Every field can have different errors. For example, this are the
forms:

username
password
etc

And you want to validate them with some class. Is this good pattern:

class Foo(object):
def validateUsername(username):
if username isn't correct:
raise ValidationError()
def validatePassword(password):
if password isn't correct:
raise ValidationError()

code:
try:
usernameError = validateUsername()
except ValidationError:
usernameError = error from exception
try:
passwordError = validatePassword()
except ValidationError:
passwordError = error from exception

So, if there wasn't any errors, usernameError and passwordError both
contains None, and there was error, both contains some string? Should
I use exception or just return None or some string without
exception?
3 Answers

Nils Oliver Kröger

3/3/2008 4:05:00 PM

0

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Hello,

I don't think it is a good pattern because you are kind of mixing
exceptions with return codes which makes the code a lot less readable
later on.

I personally would strongly opt for return codes in this case as one
would intuitively expect a function named validateThisAndThat to return
the result of a validation. This might me a simple true/false, a numeric
code with 0=OK, 1=password not correct, 2=user does not exist or a
string, whatever you need.

In my opinion, such a function should raise an exception if it is unable
to fullfill its task. For example lost connection to user database or
things like that. A function should never propagate an expected result
as an exception.

Greetings Nils

Monica Leko schrieb:
| Suppose you have some HTML forms which you would like to validate.
| Every field can have different errors. For example, this are the
| forms:
|
| username
| password
| etc
|
| And you want to validate them with some class. Is this good pattern:
|
| class Foo(object):
| def validateUsername(username):
| if username isn't correct:
| raise ValidationError()
| def validatePassword(password):
| if password isn't correct:
| raise ValidationError()
|
| code:
| try:
| usernameError = validateUsername()
| except ValidationError:
| usernameError = error from exception
| try:
| passwordError = validatePassword()
| except ValidationError:
| passwordError = error from exception
|
| So, if there wasn't any errors, usernameError and passwordError both
| contains None, and there was error, both contains some string? Should
| I use exception or just return None or some string without
| exception?

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Bruno Desthuilliers

3/3/2008 4:17:00 PM

0

Monica Leko a écrit :
> Suppose you have some HTML forms which you would like to validate.
> Every field can have different errors. For example, this are the
> forms:
>
> username
> password
> etc
>
> And you want to validate them with some class.

This is what the FormEncode package is for.

Arnaud Delobelle

3/3/2008 4:21:00 PM

0



Monica Leko wrote:

> Suppose you have some HTML forms which you would like to validate.
> Every field can have different errors. For example, this are the
> forms:
>
> username
> password
> etc
>
> And you want to validate them with some class. Is this good pattern:
[...]

You could have a look at how django [http://www.djangop...] and
how they handle forms:

http://www.djangop.../documentation/newforms/

In fact their newforms module should work fine as a standalone.
--
Arnaud