How to get the feature ID of the first feature using PyQGIS?

Question

I'm trying to automate a workflow by making a PyQGIS script, but I haven't used Python before, so it's all a rollercoaster making it work. I noticed after some vector processing on a shapefile layer that my feature IDs changed from 0, 1, 2, 3,... to 1, 2, 3, 4,... in the resulting temporary layer. The question is why? And how can I find out which applies for a certain layer in PyQGIS?

I read somewhere that shapefiles don't have persistent IDs, so I thought it might have something to do with that (SHP vs temp layer). Is that it, or is there something more to it? I want my script to accept both types of input, and automatically figure out the FID of the first feature. How do I do that?

Let's say I have the following input layer

source = self.parameterAsSource(
   parameters,
   self.INPUT,
   context

Now what? How do I retrieve the first feature ID without knowing what it is?

Answer 1

Most basic, but maybe not most elegant solution is to just break the feature iteration after the first feature using break command:

source = self.parameterAsSource(parameters, self.INPUT, context)

first_fid = None
for feat in source.getFeatures():
    first_fid = feat.id()
    break

layer.getFeatures() will always iterate in order of the feature ids if you do not add an order by statement.