Editing YAML file by Python

Question

I have a YAML file that looks like this:

# Sense 1 - name  : sense1   type  : float   value : 31  # sense 2 - name  : sense2   type  : uint32_t   value : 1488  # Sense 3 - name  : sense3   type  : int32_t   value : 0  - name  : sense4   type  : int32_t   value : 0 - name  : sense5   type  : int32_t   value : 0 - name  : sense6   type  : int32_t   value : 0 

I want to use Python to open this file, change some of the values (see above) and close the file. How can I do that ?

For instance I want to set sense2[value]=1234, keeping the YAML output the same.

Answer 1

If you care about preserving the order of your mapping keys, the comment and the white space between the elements of the root-level sequence, e.g. because this file is under revision control, then you should use ruamel.yaml (disclaimer: I am the author of that package).

Assuming your YAML document is in the file input.yaml:

import sys import ruamel.yaml  yaml = ruamel.yaml.YAML() # yaml.preserve_quotes = True with open('input.yaml') as fp:     data = yaml.load(fp) for elem in data:     if elem['name'] == 'sense2':          elem['value'] = 1234          break  # no need to iterate further yaml.dump(data, sys.stdout) 

gives:

# Sense 1 - name: sense1   type: float   value: 31  # sense 2 - name: sense2   type: uint32_t   value: 1234  # Sense 3 - name: sense3   type: int32_t   value: 0  - name: sense4   type: int32_t   value: 0 - name: sense5   type: int32_t   value: 0 - name: sense6   type: int32_t   value: 0 

This can safely be used on untrusted YAML. The (default) RoundtripLoader is a subclass of the SafeLoader even though it can handle and preserve tags (which it doesn't interpret in the dangerous way PyYAML does when enabling loading of unregistered tags).