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This article explains the concepts behind how WebAssembly works including its goals, the problems it solves, and how it runs inside the web browser's rendering engine.

WebAssembly is a new type of code that can be run in modern web browsers and provides new features and major gains in performance. It is not primarily intended to be written by hand, rather it is designed to be an effective compilation target for low-level source languages like C, C++, Rust, etc.

This has huge implications for the web platform — it provides a way to run code written in multiple languages on the web at near-native speed, with client apps running on the web that previously couldn’t have done so.

What’s more, you don’t even have to know how to create WebAssembly code to take advantage of it. WebAssembly modules can be imported into a web (or Node.js) app, exposing WebAssembly functions for use via JavaScript. JavaScript frameworks could make use of WebAssembly to confer massive performance advantages and new features while still making functionality easily available to web developers.

WebAssembly is being created as an open standard inside the W3C WebAssembly Community Group with the following goals:

Note : WebAssembly will also have uses outside web and JavaScript environments (see Non-web embeddings ).

The web platform can be thought of as having two parts:

Historically, the VM has been able to load only JavaScript. This has worked well for us as JavaScript is powerful enough to solve most problems people have on the Web today. We have run into performance problems, however, when trying to use JavaScript for more intensive use cases like 3D games, Virtual and Augmented Reality, computer vision, image/video editing, and a number of other domains that demand native performance (see ENMAYER Sexy Cover Heels Extreme High Heels Sandals Party Free Shipping 2018 146R4H1
for more ideas).

Additionally, the cost of downloading, parsing, and compiling very large JavaScript applications can be prohibitive. Mobile and other resource-constrained platforms can further amplify these performance bottlenecks.

WebAssembly is a different language from JavaScript, but it is not intended as a replacement. Instead, it is designed to complement and work alongside JavaScript, allowing web developers to take advantage of both languages' strong points:

With the advent of WebAssembly appearing in browsers, the virtual machine that we talked about earlier will now load and run two types of code — JavaScript AND WebAssembly.

The different code types can call each other as required — the wraps exported WebAssembly code with JavaScript functions that can be called normally, and WebAssembly code can import and synchronously call normal JavaScript functions. In fact, the basic unit of WebAssembly code is called a module and WebAssembly modules are symmetric in many ways to ES2015 modules.

WebAssembly key concepts

There are several key concepts needed to understand how WebAssembly runs in the browser. All of these concepts are reflected 1:1 in the .

The JavaScript API provides developers with the ability to create modules, memories, tables, and instances. Given a WebAssembly instance, JavaScript code can synchronously call its exports, which are exposed as normal JavaScript functions. Arbitrary JavaScript functions can also be synchronously called by WebAssembly code by passing in those JavaScript functions as the imports to a WebAssembly instance.

Since JavaScript has complete control over how WebAssembly code is downloaded, compiled and run, JavaScript developers could even think of WebAssembly as just a JavaScript feature for efficiently generating high-performance functions.

In the future, WebAssembly modules will be loadable just like ES2015 modules (using ), meaning that JavaScript will be able to fetch, compile, and import a WebAssembly module as easily as an ES2015module.


If any keyword argument does not correspond to a formal parameter name, a TypeError exception is raised, unless a formal parameter using the syntax **identifier is present; in this case, that formal parameter receives a dictionary containing the excess keyword arguments (using the keywords as keys and the argument values as corresponding values), or a (new) empty dictionary if there were no excess keyword arguments.

If the syntax *expression appears in the function call, expression must evaluate to an iterable. Elements from this iterable are treated as if they were additional positional arguments; if there are positional arguments x1 , …, xN , and expression evaluates to a sequence y1 , …, yM , this is equivalent to a call with M+N positional arguments x1 , …, xN , y1 , …, yM .

A consequence of this is that although the *expression syntax may appear after some keyword arguments, it is processed before the keyword arguments (and the **expression argument, if any – see below). So:

It is unusual for both keyword arguments and the *expression syntax to be used in the same call, so in practice this confusion does not arise.

If the syntax **expression appears in the function call, expression must evaluate to a mapping, the contents of which are treated as additional keyword arguments. In the case of a keyword appearing in both expression and as an explicit keyword argument, a TypeError exception is raised.

Formal parameters using the syntax *identifier or **identifier cannot be used as positional argument slots or as keyword argument names. Formal parameters using the syntax (sublist) cannot be used as keyword argument names; the outermost sublist corresponds to a single unnamed argument slot, and the argument value is assigned to the sublist using the usual tuple assignment rules after all other parameter processing is done.

A call always returns some value, possibly None , unless it raises an exception. How this value is computed depends on the type of the callable object.

If it is—

The code block for the function is executed, passing it the argument list. The first thing the code block will do is bind the formal parameters to the arguments; this is described in section Function definitions . When the code block executes a return statement, this specifies the return value of the function call.

If you only pass in a single field, you can also pass in the flat parameter. If True , this will mean the returned results are single values, rather than one-tuples. An example should make the difference clearer:

It is an error to pass in flat when there is more than one field.

You can pass named=True to get results as a HanDuEKe 2018 Women Beach Flat Flip Flop Summer Casual For Cheap Discount Clearance Real Free Shipping Cheap Quality c4Ord9ykiV

Using a named tuple may make use of the results more readable, at the expense of a small performance penalty for transforming the results into a named tuple.

If you don’t pass any values to values_list() , it will return all the fields in the model, in the order they were declared.

A common need is to get a specific field value of a certain model instance. To achieve that, use values_list() followed by a get() call:

values() and values_list() are both intended as optimizations for a specific use case: retrieving a subset of data without the overhead of creating a model instance. This metaphor falls apart when dealing with many-to-many and other multivalued relations (such as the one-to-many relation of a reverse foreign key) because the “one row, one object” assumption doesn’t hold.

For example, notice the behavior when querying across a ManyToManyField :

Authors with multiple entries appear multiple times and authors without any entries have None for the entry headline.

Similarly, when querying a reverse foreign key, None appears for entries not having any author:

Changed in Django 1.11:

Support for expressions in was added.

Changed in Django 2.0:

The parameter was added.

Returns a QuerySet that evaluates to a list of objects representing all available dates of a particular kind within the contents of the QuerySet .

field should be the name of a DateField of your model. kind should be either "year" , "month" or "day" . Each object in the result list is “truncated” to the given type .

order , which defaults to 'ASC' , should be either 'ASC' or 'DESC' . This specifies how to order the results.

Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use None as the default, and explicitly test for it in the body of the function, e.g.:

Default parameter values are evaluated from left to right when the function definition is executed.

Function call semantics are described in more detail in section Calls . A function call always assigns values to all parameters mentioned in the parameter list, either from position arguments, from keyword arguments, or from default values. If the form “ *identifier ” is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. If the form “ **identifier ” is present, it is initialized to a new ordered mapping receiving any excess keyword arguments, defaulting to a new empty mapping of the same type. Parameters after “ * ” or “ *identifier ” are keyword-only parameters and may only be passed used keyword arguments.

Parameters may have annotations of the form “ : expression ” following the parameter name. Any parameter may have an annotation even those of the form *identifier or **identifier . Functions may have “return” annotation of the form “ -> expression ” after the parameter list. These annotations can be any valid Python expression. The presence of annotations does not change the semantics of a function. The annotation values are available as values of a dictionary keyed by the parameters’ names in the __annotations__ attribute of the function object. If the annotations import from 2018 New Design Womens Fly Casual Shoes Girls Sports Sneaker Sale Visit SGtWwh
is used, annotations are preserved as strings at runtime which enables postponed evaluation. Otherwise, they are evaluated when the function definition is executed. In this case annotations may be evaluated in a different order than they appear in the source code.

It is also possible to create anonymous functions (functions not bound to a name), for immediate use in expressions. This uses lambda expressions, described in section Lambdas . Note that the lambda expression is merely a shorthand for a simplified function definition; a function defined in a “ OZZEG 2018 Diamond Slippers Shining Glitter Bling Summer Sale Footaction eQfMkXuc7
” statement can be passed around or assigned to another name just like a function defined by a lambda expression. The “ def ” form is actually more powerful since it allows the execution of multiple statements and annotations.

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