<\/div>

<\/div><\/div>

<\/p>\n

layer = iface.activeLayer()<\/p>\n

<\/div><\/div>\n\n\n\n

The first safety rule is simple:<\/p>\n\n\n

<\/div>

<\/div><\/div>

<\/p>\n

if not layer:
\nreturn<\/p>\n

<\/div><\/div>\n\n\n\n

\n
No active layer means no action should be performed.<\/p>\n<\/blockquote>\n\n\n\n
\n\n\n\n
<\/span>Checking Whether Features Are Selected<\/span><\/h2>\n\n\n\n
Before starting any processing, your plugin should verify that the user has actually selected something.<\/p>\n\n\n
<\/div>
<\/div>
<\/div><\/div>
<\/p>\n
selected_features = layer.selectedFeatures()<\/p>\n
if not selected_features:
\n# no selected features
\nreturn<\/p>\n
<\/div><\/div>\n\n\n\n
At this point, the plugin already knows whether it should:<\/p>\n\n\n\n
\n
continue processing,<\/li>\n\n\n\n
or stop gracefully.<\/li>\n<\/ul>\n\n\n\n
This simple check prevents many user errors.<\/p>\n\n\n\n
\n\n\n\n
<\/span>Processing Selected Features<\/span><\/h2>\n\n\n\n
The selectedFeatures()<\/code> method returns a list of QgsFeature<\/code> objects.<\/p>\n\n\n\n
You can therefore iterate through them easily:<\/p>\n\n\n
<\/div><\/div><\/div><\/div><\/p>\nfor feature in selected_features: \n# process each feature<\/p>\n <\/div><\/div>\n\n\n\n Each feature contains:<\/p>\n\n\n\n \nits attributes,<\/li>\n\n\n\n its geometry,<\/li>\n\n\n\n its unique identifier.<\/li>\n<\/ul>\n\n\n\nThis is everything your plugin needs to perform targeted operations.<\/p>\n\n\n\n \n\n\n\n<\/span>A Simple Example: Reading an Attribute<\/span><\/h2>\n\n\n\nOnce you have access to the selected features, you can retrieve their attribute values just as you would with any other QgsFeature<\/code>.<\/p>\n\n\n\n For example:<\/p>\n\n\n <\/div><\/div><\/div><\/div><\/p>\nfor feature in selected_features: \nvalue = feature[\u00ab\u00a0population\u00a0\u00bb]<\/p>\n <\/div><\/div>\n\n\n\n In this example, the plugin reads the value of the field named population<\/code> for each selected feature.<\/p>\n\n\n\n If the field does not exist, an error will occur.<\/p>\n\n\n\n \n This is why it is important to verify the structure of the layer before accessing attributes. As we saw in previous articles, a plugin should never assume that a field is always present.<\/p>\n<\/blockquote>\n\n\n\n \n\n\n\n<\/span>Modifying Attributes Safely<\/span><\/h2>\n\n\n\nUpdating selected features requires the layer to be in editing mode.<\/p>\n\n\n\n Before making any changes, your plugin should verify whether editing is already enabled.<\/p>\n\n\n <\/div><\/div><\/div><\/div><\/p>\nif not layer.isEditable(): \nlayer.startEditing()<\/p>\n <\/div><\/div>\n\n\n\n You can then update the selected features:<\/p>\n\n\n <\/div><\/div><\/div><\/div><\/p>\nfor feature in selected_features: \nfeature[\u00ab\u00a0status\u00a0\u00bb] = \u00ab\u00a0processed\u00a0\u00bb \nlayer.updateFeature(feature)<\/p>\n <\/div><\/div>\n\n\n\n Finally, save the changes:<\/p>\n\n\n <\/div><\/div><\/div><\/div><\/p>\nlayer.commitChanges()<\/p>\n <\/div><\/div>\n\n\n\n \n A plugin should never silently modify data without the user’s knowledge.<\/p>\n<\/blockquote>\n\n\n\n Later in this series, we will see how to request confirmation before committing changes.<\/p>\n\n\n\n \n\n\n\n<\/span>Active Selection \u2260 Filter \u2260 Temporary Layer<\/span><\/h2>\n\n\n\nQGIS provides several ways to work with a subset of data.<\/p>\n\n\n\n Although they may appear similar from the user’s perspective, they behave very differently inside a plugin.<\/p>\n\n\n\n Understanding these differences is essential for writing reliable tools.<\/p>\n\n\n\n <\/span>Active Selection<\/span><\/h3>\n\n\n\nAn active selection:<\/p>\n\n\n\n \nbelongs to the layer,<\/li>\n\n\n\n is visually highlighted,<\/li>\n\n\n\n is accessible through selectedFeatures()<\/code>,<\/li>\n\n\n\n changes whenever the user updates the selection.<\/li>\n<\/ul>\n\n\n\n\n This is the safest and most explicit mechanism for most plugins.<\/p>\n<\/blockquote>\n\n\n\n <\/span>Layer Filter<\/span><\/h3>\n\n\n\nA layer filter:<\/p>\n\n\n\n \nhides features that do not match a query,<\/li>\n\n\n\n makes the layer appear smaller,<\/li>\n\n\n\n affects what the user sees.<\/li>\n<\/ul>\n\n\n\nHowever, unless additional precautions are taken, a plugin may still access features that are hidden by the filter.<\/p>\n\n\n\n \n A poorly designed plugin can unintentionally modify features that are not visible on the screen.<\/p>\n<\/blockquote>\n\n\n\n <\/span>Temporary Layer<\/span><\/h3>\n\n\n\nA temporary layer:<\/p>\n\n\n\n \nis usually created by a processing tool,<\/li>\n\n\n\n contains a copy or subset of the original data,<\/li>\n\n\n\n is independent from the source layer,<\/li>\n\n\n\n may disappear when the project is closed.<\/li>\n<\/ul>\n\n\n\n\nTemporary layers are excellent for testing, visualization, and intermediate processing, but they are not always appropriate when updating source data.<\/p>\n<\/blockquote>\n\n\n\n \n\n\n\n<\/span>Key Takeaway<\/span><\/h2>\n\n\n\nThese three concepts are fundamentally different:<\/p>\n\n\n\n \nSelection<\/strong> = explicit user intent<\/li>\n\n\n\n Filter<\/strong> = partial view of the data<\/li>\n\n\n\n Temporary layer<\/strong> = working copy<\/li>\n<\/ul>\n\n\n\n\n In most business-oriented plugins, the active selection is the best starting point.<\/p>\n<\/blockquote>\n\n\n\n \n\n\n\n<\/span>What If No Features Are Selected?<\/span><\/h2>\n\n\n\nSooner or later, every plugin developer faces the same situation:<\/p>\n\n\n\n The user launches the tool without selecting any features.<\/p>\n\n\n\n What should the plugin do?<\/p>\n\n\n\n There are two common approaches.<\/p>\n\n\n\n <\/span>Option 1 \u2013 Block the Action<\/span><\/h3>\n\n\n\nThe safest solution is simply to stop processing and inform the user.<\/p>\n\n\n\n For example:<\/p>\n\n\n\n \nPlease select at least one feature before running this tool.<\/p>\n<\/blockquote>\n\n\n\n This approach avoids ambiguity and prevents accidental modifications.<\/p>\n\n\n\n \n In most cases, this is the safest option.<\/p>\n<\/blockquote>\n\n\n\n <\/span>Option 2 \u2013 Offer an Alternative<\/span><\/h3>\n\n\n\nAnother possibility is to ask the user what they want to do.<\/p>\n\n\n\n For example:<\/p>\n\n\n\n \nNo features are currently selected. Do you want to apply this operation to the entire layer?<\/p>\n<\/blockquote>\n\n\n\n This approach provides more flexibility while still keeping the user in control.<\/p>\n\n\n\n \n The important principle is that the plugin should never make this decision silently.<\/p>\n<\/blockquote>\n\n\n\n Always give users a choice.<\/p>\n\n\n\n \n\n\n\n<\/span>Typical Use Cases<\/span><\/h2>\n\n\n\nWorking only with selected features is essential for many common GIS tasks.<\/p>\n\n\n\n For example:<\/p>\n\n\n\n \nrecalculating an attribute for a subset of features,<\/li>\n\n\n\n correcting specific geometries,<\/li>\n\n\n\n applying business rules to a targeted group of objects,<\/li>\n\n\n\n preparing data before export,<\/li>\n\n\n\n validating a specific set of records.<\/li>\n<\/ul>\n\n\n\nIn practice, a large proportion of business-oriented plugins should work this way.<\/p>\n\n\n\n Users expect tools to operate on the features they have explicitly selected.<\/p>\n\n\n\n \n\n\n\n<\/span>Why This Matters<\/span><\/h2>\n\n\n\nRespecting the current selection is not just a technical detail.<\/p>\n\n\n\n It is a design principle.<\/p>\n\n\n\n A selection represents an explicit action performed by the user.<\/p>\n\n\n\n Ignoring it often leads to confusion, unexpected results, and loss of confidence in the tool.<\/p>\n\n\n\n By contrast, a plugin that respects the active selection behaves in a predictable and professional manner.<\/p>\n\n\n\n \n\n\n\n<\/span>Key Points to Remember<\/span><\/h2>\n\n\n\nBefore moving on, remember these essential ideas:<\/p>\n\n\n\n \nA selection represents user intent.<\/li>\n\n\n\n A well-designed plugin should respect that intent.<\/li>\n\n\n\n The required PyQGIS code is simple and easy to read.<\/li>\n\n\n\n Safety comes from good design choices, not from complex code.<\/li>\n<\/ul>\n\n\n\nMany beginner developers assume that making a plugin safer requires sophisticated programming.<\/p>\n\n\n\n In reality, the most important protection often comes from a few simple checks performed at the right time.<\/p>\n\n\n\n \n\n\n\n<\/span>What You Have Learned<\/span><\/h2>\n\n\n\nBy the end of this article, you now understand:<\/p>\n\n\n\n \nhow to retrieve the active layer,<\/li>\n\n\n\n how to detect selected features,<\/li>\n\n\n\n how to process only the selected features,<\/li>\n\n\n\n how to safely modify selected objects,<\/li>\n\n\n\n why selections, filters, and temporary layers are fundamentally different.<\/li>\n<\/ul>\n\n\n\nYour plugin can now work on a targeted subset of data rather than blindly processing an entire layer.<\/p>\n\n\n\n This is a major step toward building professional and reliable QGIS plugins.<\/p>\n\n\n\n \n\n\n\n<\/span>What’s Next?<\/span><\/h2>\n\n\n\nIn the next article, we will explore a closely related topic:<\/p>\n\n\n\n how to modify features without putting data at risk.<\/p>\n\n\n\n We will learn how to:<\/p>\n\n\n\n \nwork safely in editing mode,<\/li>\n\n\n\n commit or cancel changes,<\/li>\n\n\n\n protect user data,<\/li>\n\n\n\n build plugins that users can trust.<\/li>\n<\/ul>\n\n\n\n\nA useful plugin is not only one that performs a task correctly\u2014it is also one that protects the data it modifies.<\/p>\n<\/blockquote>\n\n\n\n \n","protected":false},"excerpt":{"rendered":"In QGIS, many operations should only be applied to a subset of the data: a specific set of features selected by the user. 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