Calling Python from an expression

You can easily call Python methods from within expressions, similar to calling Tcl expressions from Python via nuke.tcl()

To do this, you use the "python" Tcl method, for example:

[python nuke.thisNode().metadata().keys()]

While this is mostly straight-forward, there are a few things to be aware of..

Square brackets in the Python expression clash with Tcl's syntax, or should either be escaped:

[python nuke.thisNode().metadata()\['input/filename'\]]

..or enclose the Python code in { } (this is what the "Py" button in the edit-expression dialog does):

[python {nuke.thisNode().metadata['input/filename']}]

Note that if the expression errors, it returns a string containing the exception's message. The following will evaluate to a string containing "name 'fakefunction' is not defined":

[python fakefunction()]

In other words, if the Python function fails, it will not cause the Tcl expression to error. This can cause problems in some (admittedly obscure) situtations, for example, say you use a [python ...] expression to calculate the autosave filename. If the method fails for some reason (error in the script, missing import etc) - you end up with autosave files called "name 'myautosavepath' is not defined".

One last thing - it is usually far more succinct to use a standard Tcl expression, instead of calling Python. I.e my first example is far shorter when written as a Tcl expression:

1

[metadata]

While calling Python from Tcl does not seem to be noticeably slower, the Python equivalents can quickly get long and hard to read (especially if characters like [ ] need to be escaped), so try to only use this where necessary (i.e when you have existing Python code/modules you wish to use)