Possible error at usage of reverse URL in templates.

[Sebastian Piskorski]

I'd like to make sure that this is an error not the feature.

I use following code in template

<li><a href="{% url 'player-select' player.id %}">{{ player.id }} : "{{ player.name}}"</a></li>

And in urls.py everything goes fine with those rules:

1. url(r'^player/select/(?P<playerId>\d+)/$', myapp.views.player.select, name="player-select"),

result URL is: /player/select/1/

or:

2. url(r'^player/select/(?P<playerId>\d+)/?$', myapp.views.player.select, name="player-select"),

result URL is: /player/select/1

or:

3. url(r'^player/select/(?P<playerId>\d+)?/?$', myapp.views.player.select, name="player-select"),

result URL is: /player/select/1

but with this rule I get error:

4. url(r'^player/select/((?P<playerId>\d+)/)?$', neuroapp.views.player.select, name="player-select"),

error message:

Reverse for 'player-select' with arguments '(1L,)' and keyword arguments '{}' not found. 1 pattern(s) tried: ['player/select/((?P<playerId>\\d+)/)?#39;]

for me 4th pattern is pretty much the same as 3rd, but Django has other opinion. If you can, please explain me why.

[Russell Keith-Magee]

Hi Sebastian,

There is a very significant difference between the third pattern and the last - the extra set of braces. That extra set of braces means that the regex group that is being matched is 

(?P<playerId>\d+)/

That is - one or more numerical digits, identified as playerId, that *must* be followed by a slash. 

Complicating matters, that entire group is marked as optional, so the "number plus slash" group is optional on the URL.

As a result,

/player/select/

and

/player/select/1/

should match the pattern, but 

/player/select/1

will not (because it doesn't end with a slash).

In contrast, pattern 3 is trying to match: 

(?P<playerId>\d+)?/?

That is - one or more numerical digits, which is an optional construct, optionally followed by a slash. This should find a match with:

/player/select/1/

/player/select/1

/player/select//

/player/select/

I hope that helps.

Yours,

Russ Magee %-)

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