In putting together an answer to Changing Multiple Influences Simultaneously I came across the Slow python script warning in the drivers editor

Using a set up as outlined here driving a pose bone's constraint influence with the armature objects "prop" property

Using a traditional driver variable

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And again using the use_self property of drivers. Since the drivers are on on pose bones, and the id data of a pose bone is the armature object then self.id_data is that object and self.id_data["prop"] (or self.id_data.get("prop", 0)) is the value of the custom property "prop" on that object.

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However this displays the Slow Python Expression warning

What are the implications of this warning? How is it flagged?

Could be related: (possibly another question) how much of self is self... In trying to game the system with drivers tried to manipulate the self.children via a parent object driver, only to find the collection empty in the driver namespace. The method also flagged the warning

  • $\begingroup$ It's important to note that just like there's simple/slow differentiation, for slow (Python) there's also safe/unsafe. For a whitelist of allowed Python names and opcodes, see driver not working - using something outside of that list - like self - will require enabling script auto-execution. $\endgroup$ Commented May 18, 2022 at 13:51

1 Answer 1


Expressions containing:

  • variable names: only ASCII characters.
  • literals: floating point and decimal integer.
  • globals: frame
  • constants: pi, True, False
  • operators: +, -, *, /, ==, !=, <, <=, >, >=, and, or, not,
  • conditional operator A if B else C
  • standard functions: min(), max(), radians(), degrees(), abs(), fabs(), floor(), ceil(), trunc(), round(), int(), sin(), cos(), tan(), asin(), acos(), atan(), atan2(), exp(), log(), sqrt(), pow(), fmod()
  • blender provided functions: lerp(), clamp(), smoothstep()

fall into Simple expression category and:

  • are evaluated directly, which improves performance on multi-core systems
  • are evaluated even when Python script execution is disabled
  • don't get the "Slow Python expression" flag
  • $\begingroup$ Would you consider it in many cases ignorable?, eg self.location.x gets the warning, making same as a variable (similar to above) doesn't. Not using self requires editing for each object copy to change variable target. Appears then any use of self in drivers requires the auto-run python scripts ..and hence slow warning if python is enabled. Cheers. $\endgroup$
    – batFINGER
    Commented Nov 2, 2020 at 11:21
  • 1
    $\begingroup$ @batFINGER it depends, using the variable makes Blender treat it better and results in better multi-threaded performance, even though both are the same thing. If there are couple drivers like this it doesn't matter, but when there are hundreds, it may have some noticeable impact, I never encountered it though. $\endgroup$ Commented Nov 2, 2020 at 22:58

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