Combining anchor and alias in one line in triggers parserError: 'Expected <block end>, but found '<alias>'

问题

I am trying to combine anchors and aliases in order to reuse values into several containers, under different names.

I tried the following code:

FWL_GV_NANSEN: &fwl_gv_nansen
  dtype: float
  value: 2715.0

FWL_GV_E3_2: &fwl_gv_e32 *fwl_gv_nansen

the goal is simply to have another variable FWL_GV_E3_2 containing the same information than FWL_GV_NANSEN that I could refer to later on.

Just the same than defining in Python (or other):

a = 5.0
b = a
c = b

But this triggers the following error message:

yaml.parser.ParserError: while parsing a block mapping in "fwl_2.yml", line 7, column 3 expected < block end >, but found '< alias >'

Is there no way to assign the content of aliases to variable used to defined a new anchor (propagating the initial values through different variables?

PS: maybe YAML is not the best language for this since it would be trivial using python's variables for example but I have to use YAML

回答1:

The YAML node properties (i.e. tags and anchors) can only occur on "real" nodes: collections (block or flow) or scalars.

You can see from the production rules that c-ns-properties can be used only there, but section for aliases also states explicitly:

Note that an alias node must not specify any properties [...]

---

What is possible if your YAML parser gives you access to the original anchor/aliases, or some anchor-to-node/object mapping (such as my ruamel.yaml package for Python when used for round-tripping), is to use a tagged scalar:

FWL_GV_NANSEN: &fwl_gv_nansen
  dtype: float
  value: 2715.0

FWL_GV_E3_2: &fwl_gv_e32 !ref fwl_gv_nansen

with the constructor of the !ref object resolving the scalar fwl_gv_nansen, with some application specific code. Normally the anchor and alias events are resolved by the composer loader step before composing (and if so that information is no longer available).

来源：https://stackoverflow.com/questions/56564835/combining-anchor-and-alias-in-one-line-in-triggers-parsererror-expected-block