In daily website content management, we often encounter situations where certain variables may be empty (or in programming terminology, may be null) due to uncompleted content, missing data, or other reasons.nil）。If not handled properly, these variables may display as blank areas on the frontend page, which not only affects the aesthetics of the page but may also confuse visitors and reduce the professionalism of the website.defaultanddefault_if_noneThese very practical filters help us elegantly handle cases where variables are empty, ensuring the integrity of website content and user experience.

Why would a variable be 'blank', and what are its effects?

When building web pages, many content fields are optional.like the subtitle of an article, certain additional parameters of a product, avatar URLs in user comments, etc.When the data of these optional fields is not entered in the background or fails to obtain a specific value due to some logical judgment, if these variables are output directly in the template, the browser will display an empty string or nothing, resulting in ugly gaps on the page.This is like a book with missing paragraphs, which greatly reduces the reading experience.In order to avoid this situation, it is particularly important to set a reasonable default value.

defaultFilter: General and flexible default value setting

defaultFilter is a very intuitive tool when you want a variable inAny that is considered as “empty”(whether it is an empty string)"", or a number0, boolean valuefalseor represents a valueless)nil) can have an alternative value, thendefaultIt is your first choice. It checks whether the variables are in these 'empty' states, and if so, it outputs the default value you specified; otherwise, it outputs the actual content of the variable itself.

For example, you might have an article author field, and if the author information is not filled in, you would not want the page to display a blank space, but rather show 'Unknown' or 'No Author' instead. In such cases, this feature can be used: 

{{ article.author|default:"佚名" }}

Assumearticle.authorThe current string is empty, then the page will display "Unknown". Ifarticle.authorThe value ofnilit will also display "Unknown". Even ifarticle.authoris a numeric variable, the value is0,defaultAlso consider it as empty and display the default value.

default_if_noneFilter: precise processingnilValue

Withdefault,default_if_noneThe filter is moreAccurate. It will only be triggered when the variable value isnilonly provide default values when (typically in programming means a variable is not assigned or points to a null reference).

This means, if a variable's value is an empty string (""）、数字0or a boolean valuefalse,default_if_nonewill be considered asValidThe value, and display them as usual, without using the default value you have set. This is very useful in some scenarios, because empty strings, zeros orfalseIt may be a state that needs to be expressed in the business logic, rather than the true 'missing'.

For example, you have an image URL field, if the user uploads an image, the URL will be displayed; if the user deletes the image but the field is updated to an empty string""[Intentionally left blank], you might want it to display as blank (or handled by CSS) rather than displaying a default placeholder. But if this image URL field is not assigned at all,nilIf the status is, then it would be more reasonable to display a default prompt like 'No images available'.

The followingdefault_if_noneUsage examples:

{{ product.imageUrl|default_if_none:"/static/images/no-image.png" }}

In this example, ifproduct.imageUrlYesnilwill display/static/images/no-image.png. But ifproduct.imageUrlis an empty string""It will directly output.""The page may be blank, which exactly meets our expectation of "intentionally leaving it blank".

How to choose an appropriate filter?

ChoosedefaultOrdefault_if_noneThe definition of 'empty' and the requirements of the business scenario depend on you:

  • Usedefault:When you want all forms of 'empty' values (including empty strings, 0, false ornilThis applies when both contents are displayed with a default value. It is suitable for common scenarios where 'display content if available, otherwise display a placeholder'.

  • Usedefault_if_none:When you need to distinguish between "有意为空" (for example, an empty string)""or numeric0and "完全缺失" (nilWhen in these two states. If you want an empty string or zero to be considered as a valid, displayable state, and only provide a default value when the variable has not been assigned at all, thendefault_if_noneThis is a more precise choice.

By reasonably utilizing these two filters, your AnQiCMS website template will become more robust, avoiding visual chaos caused by missing data, and providing visitors with a more professional and smooth browsing experience.

Common Questions (FAQ)

Q1:defaultanddefault_if_noneCan they be used simultaneously, or is there more complex default logic?

A1: Yes, they can be combined as needed, or coordinatedifImplement more complex default value logic. For example, you may want to first check if it is.nilThen check if it is an empty string. In some cases, you can also combineifThis statement is used to precisely control. For example:

{% if article.content %}
    {{ article.content|safe }}
{% else %}
    <p>该文章暂无内容。</p>
{% endif %}

This method gives you greater control, allowing you to display different default content or structure based on different null value situations.

Q2: If I don't want to display any default text but instead hide the blank area completely (for example, an emptydivlabel), what should I do?

A2: If you want to not render the area at all when the variable is empty instead of displaying default text, the best method is to useifLogical judgment. Only when the variable has a value, will the HTML structure it is located in be rendered. For example:

{% if product.description %}
    <div class="product-description">
        {{ product.description|safe }}
    </div>
{% endif %}

So, ifproduct.descriptionis empty, the wholedivLabels will not appear on the page, thus avoiding unnecessary blank areas.

Q3: Do these filters also apply to numeric variables? If a numeric variable is0How will they handle?

A3: Yes, they are also effective for numeric variables, but the handling method will be different:

  • defaultFilter:Will convert numbers0is considered as a "null" value. If your numeric variable is0,defaultit will output the default value you set.
  • default_if_noneFilter:usually not consider a number0asnil. Therefore, if your numeric variable is0,default_if_noneThis will be output directly0without using the default value. This emphasizesdefault_if_noneonly the characteristic of a completely missing variable.nil)?

Understanding this is very important for handling default values of numeric fields such as count, price, etc., for example, when the price of the product is0Do you want to display "Free" or show it directly?0.