This may be a long shot, considering Tapestry 4.1 hasn't been updated whatsoever since 2007 or '08 or something.. But at work they have an old legacy application which uses Tapestry 4.1.
I was wondering, what is the difference between
<span jwcid="something#If" condition="ognl:something">
and
<span jwcid="#If" condition="ognl:something">
if any? Or has the something before the #If no additional purpose than to make it an unique id, which can perhaps be references elsewhere?
Here is some documentation that I've been able to find for Tapestry 4.1, but I'm unable to find an answer for my question:
http://tapestry.apache.org/tapestry4.1/
http://dev.bjmaxinfo.com/docs/tapestry/4.0.2/tapestry/apidocs/index.html
https://wiki.apache.org/tapestry/
EDIT: Ok, I've found a partial answer in the documentation I've linked above.
Understanding DirectLink URLs
The URLs generated by the DirectLink component are more complex than those generated by the PageLink component. Let's look at one:
http://localhost:8080/directlink/app?component=%24DirectLink&page=Home&service=direct&session=T
The first query parameter, component, identifies the component within the page. That %24 is "URL-ese" for a dollar sign. In Tapestry, every component ends up with a unique id within its page. If you don't provide one, Tapestry creates one from the component type, prefixed with a dollar sign. Here, our annoynmous DirectLink component was given the id $DirectLink. If you had many different DirectLinks on a page, you'd start seeing component ids such as $DirectLink_0, $DirectLink_1, etc.
You can give a component a shorter and more mneumonic id by putting the desired id before the "#" sign:
increment counter
After making that change to the template, the URL for the DirectLink component is just a bit easier to read:
http://localhost:8080/directlink/app?component=inc&page=Home&service=direct&session=T
But how does this apply to the #If, which doesn't show up in URLs?
As you said, this is just an identifier which Tapestry uses in its component model.
Here's relevant documentation from user guide: Component IDs
Related
I'm a bit confused over the difference between docstrings and comments in python.
In my class my teacher introduced something known as a 'design recipe', a set of steps that will supposedly help us students plot and organize our coding better in Python. From what I understand, the below is an example of the steps we follow - this so call design recipe (the stuff in the quotations):
def term_work_mark(a0_mark, a1_mark, a2_mark, ex_mark, midterm_mark):
''' (float, float, float, float, float) -> float
Takes your marks on a0_mark, a1_mark, a2_mark, ex_mark and midterm_mark,
calculates their respective weight contributions and sums these
contributions to deliver your overall term mark out of a maximum of 55 (This
is because the exam mark is not taken account of in this function)
>>>term_work_mark(5, 5, 5, 5, 5)
11.8
>>>term_work_mark(0, 0, 0, 0, 0)
0.0
'''
a0_component = contribution(a0_mark, a0_max_mark, a0_weight)
a1_component = contribution(a1_mark, a1_max_mark, a1_weight)
a2_component = contribution(a2_mark, a2_max_mark, a2_weight)
ex_component = contribution(ex_mark, exercises_max_mark,exercises_weight)
mid_component = contribution(midterm_mark, midterm_max_mark, midterm_weight)
return (a0_component + a1_component + a2_component + ex_component +
mid_component)
As far as I understand this is basically a docstring, and in our version of a docstring it must include three things: a description, examples of what your function should do if you enter it in the python shell, and a 'type contract', a section that shows you what types you enter and what types the function will return.
Now this is all good and done, but our assignments require us to also have comments which explain the nature of our functions, using the token '#' symbol.
So, my question is, haven't I already explained what my function will do in the description section of the docstring? What's the point of adding comments if I'll essentially be telling the reader the exact same thing?
It appears your teacher is a fan of How to Design Programs ;)
I'd tackle this as writing for two different audiences who won't always overlap.
First there are the docstrings; these are for people who are going to be using your code without needing or wanting to know how it works. Docstrings can be turned into actual documentation. Consider the official Python documentation - What's available in each library and how to use it, no implementation details (Unless they directly relate to use)
Secondly there are in-code comments; these are to explain what is going on to people (generally you!) who want to extend the code. These will not normally be turned into documentation as they are really about the code itself rather than usage. Now there are about as many opinions on what makes for good comments (or lack thereof) as there are programmers. My personal rules of thumb for adding comments are to explain:
Parts of the code that are necessarily complex. (Optimisation comes to mind)
Workarounds for code you don't have control over, that may otherwise appear illogical
I'll admit to TODOs as well, though I try to keep that to a minimum
Where I've made a choice of a simpler algorithm where a better performing (but more complex) option can go if performance in that section later becomes critical
Since you're coding in an academic setting, and it sounds like your lecturer is going for verbose, I'd say just roll with it. Use code comments to explain how you are doing what you say you are doing in the design recipe.
I believe that it's worth to mention what PEP8 says, I mean, the pure concept.
Docstrings
Conventions for writing good documentation strings (a.k.a. "docstrings") are immortalized in PEP 257.
Write docstrings for all public modules, functions, classes, and methods. Docstrings are not necessary for non-public methods, but you should have a comment that describes what the method does. This comment should appear after the def line.
PEP 257 describes good docstring conventions. Note that most importantly, the """ that ends a multiline docstring should be on a line by itself, e.g.:
"""Return a foobang
Optional plotz says to frobnicate the bizbaz first.
"""
For one liner docstrings, please keep the closing """ on the same line.
Comments
Block comments
Generally apply to some (or all) code that follows them, and are indented to the same level as that code. Each line of a block comment starts with a # and a single space (unless it is indented text inside the comment).
Paragraphs inside a block comment are separated by a line containing a single #.
Inline Comments
Use inline comments sparingly.
An inline comment is a comment on the same line as a statement. Inline comments should be separated by at least two spaces from the statement. They should start with a # and a single space.
Inline comments are unnecessary and in fact distracting if they state the obvious.
Don't do this:
x = x + 1 # Increment x
But sometimes, this is useful:
x = x + 1 # Compensate for border
Reference
https://www.python.org/dev/peps/pep-0008/#documentation-strings
https://www.python.org/dev/peps/pep-0008/#inline-comments
https://www.python.org/dev/peps/pep-0008/#block-comments
https://www.python.org/dev/peps/pep-0257/
First of all, for formatting your posts you can use the help options above the text area you type your post.
And about comments and doc strings, the doc string is there to explain the overall use and basic information of the methods. On the other hand comments are meant to give specific information on blocks or lines, #TODO is used to remind you what you want to do in future, definition of variables and so on. By the way, in IDLE the doc string is shown as a tool tip when you hover over the method's name.
Quoting from this page http://www.pythonforbeginners.com/basics/python-docstrings/
Python documentation strings (or docstrings) provide a convenient way
of associating documentation with Python modules, functions, classes,
and methods.
An object's docsting is defined by including a string constant as the
first statement in the object's definition.
It's specified in source code that is used, like a comment, to
document a specific segment of code.
Unlike conventional source code comments the docstring should describe
what the function does, not how.
All functions should have a docstring
This allows the program to inspect these comments at run time, for
instance as an interactive help system, or as metadata.
Docstrings can be accessed by the __doc__ attribute on objects.
Docstrings can be accessed through a program (__doc__) where as inline comments cannot be accessed.
Interactive help systems like in bpython and IPython can use docstrings to display the docsting during the development. So that you dont have to visit the program everytime.
We are using SQLite property database to get information about object's properties. The problem we are facing with is property identification. For each property we have:
id: looks like it is just an autoincremental id and it can be used to
identify property between two versions of the same model
name: for .rvt models it is mostly unique, but there are some duplications
sometimes (for example the same property with different flags,
readonly and not); for .ifc files it is not unique at all
category + displayName: the problem with this
fields is that after using design automation API (to change and
re-save model) category/displayName can be translated into English
(but initially they can be, for example, in German)
Now we are using the combination of 'name + category + displayName' to identify the property, but it is not the solution, because this combination still is not unique in some rare cases (it is data lost for us) and it doesn't solve the problem with property names translation using Design Automation API.
Any ideas how to identify properties will be helpful! Thanks
For a given element, the externalId should be unique within that model (at the model level, the urn is unique). There is no unique identification for properties.
I'm interested in understand this workflow better, are you able to have a quick talk? Please use this link to book a time.
I read quite a lot about the visitor pattern and its supposed advantages. To me however it seems they are not that much advantages when applied in practice:
"Convenient" and "elegant" seems to mean lots and lots of boilerplate code
Therefore, the code is hard to follow. Also 'accept'/'visit' is not very descriptive
Even uglier boilerplate code if your programming language has no method overloading (i.e. Vala)
You cannot in general add new operations to an existing type hierarchy without modification of all classes, since you need new 'accept'/'visit' methods everywhere as soon as you need an operation with different parameters and/or return value (changes to classes all over the place is one thing this design pattern was supposed to avoid!?)
Adding a new type to the type hierarchy requires changes to all visitors. Also, your visitors cannot simply ignore a type - you need to create an empty visit method (boilerplate again)
It all just seems to be an awful lot of work when all you want to do is actually:
// Pseudocode
int SomeOperation(ISomeAbstractThing obj) {
switch (type of obj) {
case Foo: // do Foo-specific stuff here
case Bar: // do Bar-specific stuff here
case Baz: // do Baz-specific stuff here
default: return 0; // do some sensible default if type unknown or if we don't care
}
}
The only real advantage I see (which btw i haven't seen mentioned anywhere): The visitor pattern is probably the fastest method to implement the above code snippet in terms of cpu time (if you don't have a language with double dispatch or efficient type comparison in the fashion of the pseudocode above).
Questions:
So, what advantages of the visitor pattern have I missed?
What alternative concepts/data structures could be used to make the above fictional code sample run equally fast?
For as far as I have seen so far there are two uses / benefits for the visitor design pattern:
Double dispatch
Separate data structures from the operations on them
Double dispatch
Let's say you have a Vehicle class and a VehicleWasher class. The VehicleWasher has a Wash(Vehicle) method:
VehicleWasher
Wash(Vehicle)
Vehicle
Additionally we also have specific vehicles like a car and in the future we'll also have other specific vehicles. For this we have a Car class but also a specific CarWasher class that has an operation specific to washing cars (pseudo code):
CarWasher : VehicleWasher
Wash(Car)
Car : Vehicle
Then consider the following client code to wash a specific vehicle (notice that x and washer are declared using their base type because the instances might be dynamically created based on user input or external configuration values; in this example they are simply created with a new operator though):
Vehicle x = new Car();
VehicleWasher washer = new CarWasher();
washer.Wash(x);
Many languages use single dispatch to call the appropriate function. Single dispatch means that during runtime only a single value is taken into account when determining which method to call. Therefore only the actual type of washer we'll be considered. The actual type of x isn't taken into account. The last line of code will therefore invoke CarWasher.Wash(Vehicle) and NOT CarWasher.Wash(Car).
If you use a language that does not support multiple dispatch and you do need it (I can honoustly say I have never encountered such a situation though) then you can use the visitor design pattern to enable this. For this two things need to be done. First of all add an Accept method to the Vehicle class (the visitee) that accepts a VehicleWasher as a visitor and then call its operation Wash:
Accept(VehicleWasher washer)
washer.Wash(this);
The second thing is to modify the calling code and replace the washer.Wash(x); line with the following:
x.Accept(washer);
Now for the call to the Accept method the actual type of x is considered (and only that of x since we are assuming to be using a single dispatch language). In the implementation of the Accept method the Wash method is called on the washer object (the visitor). For this the actual type of the washer is considered and this will invoke CarWasher.Wash(Car). By combining two single dispatches a double dispatch is implemented.
Now to eleborate on your remark of the terms like Accept and Visit and Visitor being very unspecific. That is absolutely true. But it is for a reason.
Consider the requirement in this example to implement a new class that is able to repair vehicles: a VehicleRepairer. This class can only be used as a visitor in this example if it would inherit from VehicleWasher and have its repair logic inside a Wash method. But that ofcourse doesn't make any sense and would be confusing. So I totally agree that design patterns tend to have very vague and unspecific naming but it does make them applicable to many situations. The more specific your naming is, the more restrictive it can be.
Your switch statement only considers one type which is actually a manual way of single dispatch. Applying the visitor design pattern in the above way will provide double dispatch.
This way you do not necessarily need additional Visit methods when adding additional types to your hierarchy. Ofcourse it does add some complexity as it makes the code less readable. But ofcourse all patterns come at a price.
Ofcourse this pattern cannot always be used. If you expect lots of complex operations with multiple parameters then this will not be a good option.
An alternative is to use a language that does support multiple dispatch. For instance .NET did not support it until version 4.0 which introduced the dynamic keyword. Then in C# you can do the following:
washer.Wash((dynamic)x);
Because x is then converted to a dynamic type its actual type will be considered for the dispatch and so both x and washer will be used to select the correct method so that CarWasher.Wash(Car) will be called (making the code work correctly and staying intuitive).
Separate data structures and operations
The other benefit and requirement is that it can separate the data structures from the operations. This can be an advantage because it allows new visitors to be added that have there own operations while it also allows data structures to be added that 'inherit' these operations. It can however be only applied if this seperation can be done / makes sense. The classes that perform the operations (the visitors) do not know the structure of the data structures nor do they have to know that which makes code more maintainable and reusable. When applied for this reason the visitors have operations for the different elements in the data structures.
Say you have different data structures and they all consist of elements of class Item. The structures can be lists, stacks, trees, queues etc.
You can then implement visitors that in this case will have the following method:
Visit(Item)
The data structures need to accept visitors and then call the Visit method for each Item.
This way you can implement all kinds of visitors and you can still add new data structures as long as they consist of elements of type Item.
For more specific data structures with additional elements (e.g. a Node) you might consider a specific visitor (NodeVisitor) that inherits from your conventional Visitor and have your new data structures accept that visitor (Accept(NodeVisitor)). The new visitors can be used for the new data structures but also for the old data structures due to inheritence and so you do not need to modify your existing 'interface' (the super class in this case).
In my personal opinion, the visitor pattern is only useful if the interface you want implemented is rather static and doesn't change a lot, while you want to give anyone a chance to implement their own functionality.
Note that you can avoid changing everything every time you add a new method by creating a new interface instead of modifying the old one - then you just have to have some logic handling the case when the visitor doesn't implement all the interfaces.
Basically, the benefit is that it allows you to choose the correct method to call at runtime, rather than at compile time - and the available methods are actually extensible.
For more info, have a look at this article - http://rgomes-info.blogspot.co.uk/2013/01/a-better-implementation-of-visitor.html
By experience, I would say that "Adding a new type to the type hierarchy requires changes to all visitors" is an advantage. Because it definitely forces you to consider the new type added in ALL places where you did some type-specific stuff. It prevents you from forgetting one....
This is an old question but i would like to answer.
The visitor pattern is useful mostly when you have a composite pattern in place in which you build a tree of objects and such tree arrangement is unpredictable.
Type checking may be one thing that a visitor can do, but say you want to build an expression based on a tree that can vary its form according to a user input or something like that, a visitor would be an effective way for you to validate the tree, or build a complex object according to the items found on the tree.
The visitor may also carry an object that does something on each node it may find on that tree. this visitor may be a composite itself chaining lots of operations on each node, or it can carry a mediator object to mediate operations or dispatch events on each node.
You imagination is the limit of all this. you can filter a collection, build an abstract syntax tree out of an complete tree, parse a string, validate a collection of things, etc.
I'm writing a web application (in Python, not that it matters). One of the features is that people can leave comments on things. I have a class for comments, basically like so:
class Comment:
user = ...
# other stuff
where user is an instance of another class,
class User:
name = ...
# other stuff
And of course in my template, I have
<div>${comment.user.name}</div>
Problem: Let's say I allow people to post comments anonymously. In that case comment.user is None (undefined), and of course accessing comment.user.name is going to raise an error. What's the best way to deal with that? I see three possibilities:
Use a conditional in the template to test for that case and display something different. This is the most versatile solution, since I can change the way anonymous comments are displayed to, say, "Posted anonymously" (instead of "Posted by ..."), but I've often been told that templates should be mindless display machines and not include logic like that. Also, other people might wind up writing alternate templates for the same application, and I feel like I should be making things as easy as possible for the template writer.
Implement an accessor method for the user property of a Comment that returns a dummy user object when the real user is undefined. This dummy object would have user.name = 'Anonymous' or something like that and so the template could access it and print its name with no error.
Put an actual record in my database corresponding to a user with user.name = Anonymous (or something like that), and just assign that user to any comment posted when nobody's logged in. I know I've seen some real-world systems that operate this way. (phpBB?)
Is there a prevailing wisdom among people who write these sorts of systems about which of these (or some other solution) is the best? Any pitfalls I should watch out for if I go one way vs. another? Whoever gives the best explanation gets the checkmark.
I'd go with the first option, using an if switch in the template.
Consider the case of localization: You'll possibly have different templates for each language. You can easily localize the "anonymous" case in the template itself.
Also, the data model should have nothing to do with the output side. What would you do in the rest of the code if you wanted to test whether a user has a name or not? Check for == 'Anonymous' each time?
The template should indeed only be concerned with outputting data, but that doesn't mean it has to consist solely of output statements. You usually have some sort of if user is logged in, display "Logout", otherwise display "Register" and "Login" case in the templates. It's almost impossible to avoid these.
Personally, I like for clean code, and agree that templates should not have major logic. So in my implementations I make sure that all values have "safe" default values, typically a blank string, pointer to a base class or equivalent. That allows for two major improvements to the code, first that you don't have to constantly test for null or missing values, and you can output default values without too much logic in your display templates.
So in your situation, making a default pointer to a base value sounds like the best solution.
Your 3rd option: Create a regular User entity that represents an anonymous user.
I'm not a fan of None for database integrity reasons.
This question bugged me for years now and can't seem to find good solution still. I working in PHP and Java but it sounds like this maybe language-agnostic :)
Say we have a standard status reference table that holds status ids for some kind of entity. Further let's assume the table will have just 5 values, and will remain like this for a long time, maybe edited occasionally with addition of a new status.
When you fetch a row and need to see what status it is you have 2 options(as I see it at least) - put it straight ID values(magic numbers that is) or use a named constant. Latter seem much cleaner, the question though is where those named constants should leave? In a model class? In a class that uses this particular constant? Somewhere else?
It sounds like what you're wanting to do is an enumerated value.
This is a value that has a literal name mapped to a constant value, this would be something like
Statusone = 1
Statustwo = 2
Then anywhere in your program you could refrenece statusone which the compiler would see as 1.
I'm not sure if this exists in php but I'm pretty sure it does in java
EDIT In response to some comments
I would typically put enumerated values in some kind of global namespace, or if you only need them when you are using that class spefically you can put them at the class level.