d5c50fc366
This PR: * Splits the `Gotchas` page into several sub-sections. This was getting too big and hard to navigate. * Adds some information regarding Python instances life-time of objects wrapping Blender internal data. * Adds some information about usage of constructors and destructors for sub-classes of Blender-defined types. Pull Request: https://projects.blender.org/blender/blender/pulls/129814
70 lines
3.0 KiB
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70 lines
3.0 KiB
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***************
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Using Operators
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***************
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.. _using_operators:
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Blender's operators are tools for users to access, that can be accessed with Python too which is very useful.
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Still operators have limitations that can make them cumbersome to script.
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The main limits are:
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- Can't pass data such as objects, meshes or materials to operate on (operators use the context instead).
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- The return value from calling an operator is the success (if it finished or was canceled),
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in some cases it would be more logical from an API perspective to return the result of the operation.
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- Operators' poll function can fail where an API function would raise an exception giving details on exactly why.
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Why does an operator's poll fail?
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=================================
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When calling an operator it gives an error like this:
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>>> bpy.ops.action.clean(threshold=0.001)
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RuntimeError: Operator bpy.ops.action.clean.poll() failed, context is incorrect
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Which raises the question as to what the correct context might be?
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Typically operators check for the active area type, a selection or active object they can operate on,
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but some operators are more strict when they run.
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In most cases you can figure out what context an operator needs
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by examining how it's used in Blender and thinking about what it does.
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If you're still stuck, unfortunately, the only way to eventually know what is causing the error is
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to read the source code for the poll function and see what it is checking.
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For Python operators it's not so hard to find the source
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since it's included with Blender and the source file and line is included in the operator reference docs.
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Downloading and searching the C code isn't so simple,
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especially if you're not familiar with the C language but by searching the operator name or description
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you should be able to find the poll function with no knowledge of C.
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.. note::
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Blender does have the functionality for poll functions to describe why they fail,
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but it's currently not used much, if you're interested to help improve the API
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feel free to add calls to :class:`bpy.types.Operator.poll_message_set` (``CTX_wm_operator_poll_msg_set`` in C)
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where it's not obvious why poll fails, e.g:
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>>> bpy.ops.gpencil.draw()
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RuntimeError: Operator bpy.ops.gpencil.draw.poll() Failed to find Grease Pencil data to draw into
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The operator still doesn't work!
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================================
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Certain operators in Blender are only intended for use in a specific context,
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some operators for example are only called from the properties editor where they check the current material,
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modifier or constraint.
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Examples of this are:
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- :mod:`bpy.ops.texture.slot_move`
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- :mod:`bpy.ops.constraint.limitdistance_reset`
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- :mod:`bpy.ops.object.modifier_copy`
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- :mod:`bpy.ops.buttons.file_browse`
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Another possibility is that you are the first person to attempt to use this operator
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in a script and some modifications need to be made to the operator to run in a different context.
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If the operator should logically be able to run but fails when accessed from a script
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it should be reported to the bug tracker.
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