Python - using a string in for in statement?

Question

so i know this is a bit of a workaround and theres probably a better way to do this, but heres the deal. Ive simplified the code from where tis gathering this info from and just given solid values.

curSel = nuke.selectedNodes()
knobToChange = "label"
codeIn = "[value in]"

kcPrefix = "x"
kcStart = "['"
kcEnd = "']"

changerString = kcPrefix+kcStart+knobToChange+kcEnd


for x in curSel:
    changerString.setValue(codeIn)

But i get the error i figured i would - which is that a string has no attribute "setValue"

its because if i just type x['label'] instead of changerString, it works, but even though changer string says the exact same thing, its being read as a string instead of code.

Any ideas?

Might be able to do for x in curSel: then for key, value in enumerate(x): ... Could you give some examples of curSel values? — bozdoz
– bozdoz, Commented Jan 7, 2013 at 16:04
Why would you want to set a single string's value to the same thing n times anyway? What are you actually trying to accomplish here? — Wooble
– Wooble, Commented Jan 7, 2013 at 16:06
Its not a single item... im ina program with Many nodes selected and a for loop is how you cycle through to change everything in this. I want to change multiple to this same value beacuse [value in' (which is what im changing it to) will affect each item differently. — aescript
– aescript, Commented Jan 7, 2013 at 16:07
do you literally want to put [value in] in each node's label slot, or am I misreading this? — tanantish
– tanantish, Commented Jan 7, 2013 at 16:07

mgilson · Accepted Answer · 2013-01-07 16:39:33Z

5

It looks like you're looking for something to evaluate the string into a python object based on your current namespace. One way to do that would be to use the globals dictionary:

globals()['x']['label'].setValue(...)

In other words, globals()['x']['label'] is the same thing as x['label'].

Or to spell it out explicitly for your case:

globals()[kcPrefix][knobToChange].setValue(codeIn)

Others might suggest eval:

eval('x["label"]').setValue(...)  #insecure and inefficient

but globals is definitely a better idea here.

Finally, usually when you want to do something like this, you're better off using a dictionary or some other sort of data structure in the first place to keep your data more organized

edited Jan 7, 2013 at 16:39

answered Jan 7, 2013 at 16:06

mgilson

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13 Comments

Wooble Over a year ago

+1 for the last sentence. Neither is a good idea in 99% of cases.

aescript Over a year ago

No this wont work , the reason im doing it the other way is its based off fo user input... if i just wanted to type x.['label'] i could so so.

aescript Over a year ago

basically what im saying is it HAS to use changerString to get the x['label'] in there, because label could change to anything, based on user input.

mgilson Over a year ago

@user1917081 -- Then why not: globals()['x'][user_input].setValue()? I'm not sure I understand your constraint ...

aescript Over a year ago

Its not so much a contraint as im new to python and dont understand what glboals is doing.

|

tanantish · Accepted Answer · 2013-01-07 18:12:29Z

2

Righto, there's two things you're falling afoul of. Firstly, in your original code where you are trying to do the setValue() call on a string you're right in that it won't work. Ideally use one of the two calls (x.knob('name_of_the_knob') or x['name_of_the_knob'], whichever is consistent with your project/facility/personal style) to get and set the value of the knob object.

From the comments, your code would look like this (my comments added for other people who aren't quite as au fait with Nuke):

# select all the nodes
curSel = nuke.selectedNodes() 

# nuke.thisNode() returns the script's context 
# i.e. the node from which the script was invoked
knobToChange = nuke.thisNode()['knobname'].getValue() 
codeIn = nuke.thisNode()['codeinput'].getValue() 

for x in curSel: 
    x.knob(knobToChange).setValue(codeIn)

Using this sample UI with the values in the two fields as shown and the button firing off the script...

sample UI with the relevant buttons

...this code is going to give you an error message of 'Nothing is named "foo"' when you execute it because the .getValue() call is actually returning you the evaluated result of the knob - which is the error message as it tries to execute the TCL [value foo], and finds that there isn't any object named foo.

What you should ideally do is instead invoke .toScript() which returns the raw text.

# select all the nodes
curSel = nuke.selectedNodes() 

# nuke.thisNode() returns the script's context 
# i.e. the node from which the script was invoked
knobToChange = nuke.thisNode()['knobname'].toScript() 
codeIn = nuke.thisNode()['codeinput'].toScript() 

for x in curSel: 
    x.knob(knobToChange).setValue(codeIn)

You can sidestep this problem as you've noted by building up a string, adding in square brackets etc etc as per your original code, but yes, it's a pain, a maintenance nightmare, and starting to go down that route of building objects up from strings (which @mgilson explains how to do in both a globals() or eval() method)

For those who haven't had the joy of working with Nuke, here's a small screencap that may (or may not..) provide more context:

sample screenie from Nuke showing the node graph and the properties dialog box

edited Jan 7, 2013 at 18:12

answered Jan 7, 2013 at 16:23

tanantish

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4 Comments

mgilson Over a year ago

To be clear -- I'm not really encouraging the "build" a string and convert to an object approach (eval). I'm not really even encouraging the "take the object's name and use it to get a handle on the object" approach (globals) -- Though that's much better than eval. I'm encouraging the "use a proper data-structure" approach :)

tanantish Over a year ago

@mgilson My bad. Rewrote the answer (since we now have a better handle on the issue) and made it a bit clearer you weren't suggesting the globals or eval methods as good ideas :)

aescript Over a year ago

Thanks man this cleans a lot up, and i was unaware of the toScript() command! that will come in very handy.

tanantish Over a year ago

it's not exactly jumping out at you from the API reference, but if you know where to look (and make the connection that what you enter in as text knob is called an 'EvalString_Knob' object..) it sorta helps :)

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