Only truncate --output-file after enriching

[japsu]

Consider the following files:

# a.yaml
a: 1
b: 2

# a.in.yaml – template
!Merge
  - !Include a.yaml
  - !Include a.update.yaml

# a.update.yaml – an incremental update to a.yaml
b: 3

Now, if you run emrichen --output-file a.yaml a.in.yaml, the resulting a.yaml will only contain

b: 3

instead of

a: 1
b: 3

because --output-file a.yaml is opened at application startup with w mode, truncating the file, and !Include a.yaml enriches to {}.

I would like to change this so that !Include a.yaml here would enrich to the original contents of a.yaml, and a.yaml would be opened for writing only after enriching the template.

[akx]

Absolutely makes sense. To avoid having to buffer the output to memory before writing, maybe the file should be opened with w+, sought to the beginning and then truncated at the start of write?