yamlpath 2.3.1

Read and change YAML/Compatible data using powerful, intuitive, command-line friendly syntax

YAML Path and Command-Line Tools

Contents:

1. Introduction
2. Illustration
3. Installing
4. Supported YAML Path Segments
5. Based on ruamel.yaml and Python 3
6. The Files of This Project
   1. Command-Line Tools
   2. Libraries
7. Basic Usage
   1. Basic Usage: Command-Line Tools
      1. Rotate Your EYAML Keys
      2. Get a YAML Value
      3. Search For YAML Paths
      4. Change a YAML Value
   2. Basic Usage: Libraries
      1. Initialize ruamel.yaml and These Helpers
      2. Searching for YAML Nodes
      3. Changing Values

Introduction

This project presents and utilizes YAML Paths, which are a powerful, intuitive means of identifying one or more nodes within YAML, EYAML, or compatible data structures like JSON. Both dot-notation (inspired by Hiera) and forward-slash-notation (influenced by XPath) are supported. The libraries (modules) and several command-line tool implementations are provided. With these, you can build YAML Path support right into your own application or easily use its capabilities right away from the command-line to retrieve or update YAML/Compatible data.

This implementation of YAML Path is a query language in addition to a node descriptor. With it, you can describe or select a single precise node or search for any number of nodes that match some criteria. Keys, values, and elements can all be searched at any number of levels within the data structure using the same query. Collectors can also be used to gather and further select from otherwise disparate parts of the source data.

Illustration

To illustrate some of these concepts, consider these samples:

---
hash:
  child_attr:
    key: 5280

This value, 5280, can be identified via YAML Path as any of:

1. hash.child_attr.key (dot-notation)
2. hash.child_attr[.=key] (search all child keys for one named, key, and yield its value)
3. /hash/child_attr/key (same as 1 but in forward-slash notation)
4. /hash/child_attr[.=key] (same as 2 but in forward-slash notation)

---
aliases:
  - &first_anchor Simple string value

With YAML Path, you can select this anchored value by any of these equivalent expressions:

1. aliases[0] (explicit array element number)
2. aliases.0 (implicit array element number in dot-notation)
3. aliases[&first_anchor] (search by Anchor name)
4. aliases[.^Simple] (search for any elements starting with "Simple")
5. aliases[.%string] (search for any elements containing "string")
6. aliases[.$value] (search for any elements ending with "value")
7. aliases[.=~/^(\b[Ss][a-z]+\s){2}[a-z]+$/] (search for any elements matching a complex Regular Expression, which happens to match the example)
8. /aliases[0] (same as 1 but in forward-slash notation)
9. /aliases/0 (same as 2 but in forward-slash notation)
10. /aliases[&first_anchor] (same as 3 but in forward-slash notation)

---
users:
  - name: User One
    password: ENC[PKCS7,MIIBiQY...Jk==]
    roles:
      - Writers
  - name: User Two
    password: ENC[PKCS7,MIIBiQY...vF==]
    roles:
      - Power Users
      - Editors

With an example like this, YAML Path enables:

• selection of single nodes: /users/0/roles/0 = Writers
• all children nodes of any given parent: /users/1/roles = ["Power Users", "Editors"]
• searching by a child attribute: /users[name="User One"]/password = Some decrypted value, provided you have the appropriate EYAML keys
• pass-through selections against arrays-of-hashes: /users/roles = ["Writers"]\n["Power Users", "Editors"] (each user's list of roles are a seperate result)
• collection of disparate results: (/users/name) = ["User One", "User Two"] (all names appear in a single result instead of one per line)

For a deeper exploration of YAML Path's capabilities, please visit the project Wiki.

Supported YAML Path Segments

A YAML Path segment is the text between seperators which identifies a parent or leaf node within the data structure. For dot-notation, a path like hash.key identifies two segments: hash (a parent node) and key (a leaf node). The same path in forward-slash notation would be: /hash/key.

YAML Path understands these segment types:

• Top-level Hash key selection: key
• Explicit top-level array element selection: [#] where # is the zero-based element number; # can also be negative, causing the element to be selected from the end of the Array
• Implicit array element selection or numbered hash key selection: # where # is the 0-based element number or exact name of a hash key which is itself a number
• Top-level (Hash) Anchor lookups: &anchor_name (the & is required to indicate you are seeking an Anchor by name)
• Hash sub-keys: hash.child.key or /hash/child/key
• Demarcation for dotted Hash keys: hash.'dotted.child.key' or hash."dotted.child.key" (not necessary when using forward-slash notation, /hash/dotted.child.key)
• Named Array element selection: array[#], array.#, /array[#], or /array/# where array is the name of the Hash key containing Array data and # is the 0-based element number
• Anchor lookups in named Arrays: array[&anchor_name] where array is the name of the Hash key containing Array data and both of the [] pair and & are required to indicate you are seeking an Anchor by name within an Array
• Array slicing: array[start#:stop#] where start# is the first inclusive, zero-based element and stop# is the last exclusive element to select; either or both can be negative, causing the elements to be selected from the end of the Array; when start# and stop# are identical, it is the same as array[start#]
• Hash slicing: hash[min:max] where min and max are alphanumeric terms between which the Hash's keys are compared
• Escape symbol recognition: hash.dotted\.child\.key, /hash/whacked\/child\/key, and keys_with_\\slashes
• Hash attribute searches (which can return zero or more matches):
  • Exact match: hash[name=admin]
  • Starts With match: hash[name^adm]
  • Ends With match: hash[name$min]
  • Contains match: hash[name%dmi]
  • Less Than match: hash[access_level<500]
  • Greater Than match: hash[access_level>0]
  • Less Than or Equal match: hash[access_level<=100]
  • Greater Than or Equal match: hash[access_level>=0]
  • Regular Expression matches: hash[access_level=~/^\D+$/] (the / Regular Expression delimiter can be substituted for any character you need, except white-space; note that / does not interfere with forward-slash notation and it does not need to be escaped because the entire search expression is contained within a [] pair)
  • Invert any match with !, like: hash[name!=admin] or even hash[!name=admin] (the former syntax is used when YAML Paths are stringified but both forms are equivalent)
  • Demarcate and/or escape expression operands, like: hash[full\ name="Some User\'s Name"] (note that embedded, single ' and " must be escaped lest they be deemed unmatched demarcation pairings)
  • Multi-level matching: hash[name%admin].pass[encrypted!^ENC\[] or /hash[name%admin]/pass[encrypted!^ENC\[]
• Array element searches with all of the search methods above via . (yields any matching elements): array[.>9000]
• Hash key-name searches with all of the search methods above via . (yields their values, not the keys themselves): hash[.^app_]
• Array-of-Hashes Pass-Through Selection: Omit a selector for the elements of an Array-of-Hashes and all matching Hash attributes at that level will be yielded (or searched when there is more to the path). For example, warriors[1].power_level or /warriors[1]/power_level will return the power_level attribute of only the second Hash in an Array-of-Hashes while warriors.power_level or /warriors/power_level will return the power_level attribute of every Hash in the same Array-of-Hashes. Of course these results can be filtered in multiple ways, like warriors[power_level>9000], /warriors[power_level>9000], warriors.power_level[.>9000], and /warriors/power_level[.>9000] all yield only the power_level from all warriors with power_levels over 9,000 within the same array of warrior hashes.
• Collectors: Placing any portion of the YAML Path within parenthesis defines a virtual list collector, like (YAML Path); concatenation and exclusion operators are supported -- + and -, respectively -- along with nesting, like (...)-((...)+(...))
• Complex combinations: some::deep.hierarchy[with!=""].'any.valid'[.=~/(yaml|json)/][data%structure].or.complexity[4].2 or /some::deep/hierarchy[with!=""]/any.valid[.=~/(yaml|json)/][data%structure]/or/complexity[4]/2

This implementation of YAML Path encourages creativity. Use whichever notation and segment types that make the most sense to you in each application.

Installing

This project requires Python 3. It is tested against Pythons 3.6 through 3.8. Most operating systems and distributions have access to Python 3 even if only Python 2 -- or no Python, at all -- came pre-installed. It is generally safe to have more than one version of Python on your system at the same time, especially when using virtual Python environments.

Each published version of this project can be installed from PyPI using pip. Note that on systems with more than one version of Python, you will probably need to use pip3, or equivalent (e.g.: Cygwin users may need to use pip3.6).

pip3 install yamlpath

EYAML support is entirely optional. You do not need EYAML to use YAML Path. That YAML Path supports EYAML is a service to a substantial audience: Puppet users. At the time of this writing, EYAML (classified as a Hiera back-end/plug-in) is available only as a Ruby Gem. That said, it provides a command-line tool, eyaml, which can be employed by this otherwise Python project. To enjoy EYAML support, install compatible versions of ruby and rubygems, then execute:

gem install hiera-eyaml

If this puts the eyaml command on your system PATH, nothing more need be done apart from generating or obtaining your encryption keys. Otherwise, you can tell YAML Path library and tools where to find the eyaml command.

Based on ruamel.yaml and Python 3

In order to support the best available YAML editing capability (so called, round-trip editing with support for comment preservation), this project is based on ruamel.yaml for Python 3. While ruamel.yaml is based on PyYAML -- Python's "standard" YAML library -- ruamel.yaml is objectively better than PyYAML, which lacks critical round-trip editing capabilities as well as up-to-date YAML/Compatible data parsing capabilities (at the time of this writing).

Should PyYAML ever merge with -- or at least, catch up with -- ruamel.yaml, this project can be (lightly) adapted to depend on it, instead. These conversations may offer some insight into when or whether this might happen:

• Is this time to pass the baton?
• Rebase off ruamel? - many new valuable features

The Files of This Project

This repository contains:

1. Generally-useful Python library files. These contain the reusable core of this project's YAML Path capabilities.
2. Some implementations of those libraries, exhibiting their capabilities and simple-to-use APIs as command-line tools.
3. Various support, documentation, and build files.

Command-Line Tools

This project provides some command-line tool implementations which utilize YAML Path. For some use-case examples of these tools, see below.

The supplied command-line tools include:

• eyaml-rotate-keys

usage: eyaml-rotate-keys [-h] [-V] [-d | -v | -q] [-b] [-x EYAML]
                         -i OLDPRIVATEKEY -c OLDPUBLICKEY
                         -r NEWPRIVATEKEY -u NEWPUBLICKEY
                         YAML_FILE [YAML_FILE ...]

Rotates the encryption keys used for all EYAML values within a set of YAML
files, decrypting with old keys and re-encrypting using replacement keys.

positional arguments:
  YAML_FILE             one or more YAML files containing EYAML values

optional arguments:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit
  -d, --debug           output debugging details
  -v, --verbose         increase output verbosity
  -q, --quiet           suppress all output except errors
  -b, --backup          save a backup of each modified YAML_FILE with an extra
                        .bak file-extension
  -x EYAML, --eyaml EYAML
                        the eyaml binary to use when it isn't on the PATH

EYAML_KEYS:
  All key arguments are required

  -r NEWPRIVATEKEY, --newprivatekey NEWPRIVATEKEY
                        the new EYAML private key
  -u NEWPUBLICKEY, --newpublickey NEWPUBLICKEY
                        the new EYAML public key
  -i OLDPRIVATEKEY, --oldprivatekey OLDPRIVATEKEY
                        the old EYAML private key
  -c OLDPUBLICKEY, --oldpublickey OLDPUBLICKEY
                        the old EYAML public key

Any YAML_FILEs lacking EYAML values will not be modified (or backed up, even
when -b/--backup is specified).

• yaml-get

usage: yaml-get [-h] [-V] -p YAML_PATH
                [-t ['.', '/', 'auto', 'dot', 'fslash']] [-x EYAML]
                [-r PRIVATEKEY] [-u PUBLICKEY] [-d | -v | -q]
                YAML_FILE

Retrieves one or more values from a YAML file at a specified YAML Path. Output
is printed to STDOUT, one line per result. When a result is a complex data-
type (Array or Hash), a JSON dump is produced to represent it. EYAML can be
employed to decrypt the values.

positional arguments:
  YAML_FILE             the YAML file to query

optional arguments:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit
  -t ['.', '/', 'auto', 'dot', 'fslash'], --pathsep ['.', '/', 'auto', 'dot', 'fslash']
                        indicate which YAML Path seperator to use when
                        rendering results; default=dot
  -d, --debug           output debugging details
  -v, --verbose         increase output verbosity
  -q, --quiet           suppress all output except errors

required settings:
  -p YAML_PATH, --query YAML_PATH
                        YAML Path to query

EYAML options:
  Left unset, the EYAML keys will default to your system or user defaults.
  Both keys must be set either here or in your system or user EYAML
  configuration file when using EYAML.

  -x EYAML, --eyaml EYAML
                        the eyaml binary to use when it isn't on the PATH
  -r PRIVATEKEY, --privatekey PRIVATEKEY
                        EYAML private key
  -u PUBLICKEY, --publickey PUBLICKEY
                        EYAML public key

For more information about YAML Paths, please visit
https://github.com/wwkimball/yamlpath.

• yaml-paths

usage: yaml-paths [-h] [-V] -s EXPRESSION [-c EXPRESSION] [-d | -v | -q] [-m]
                  [-L] [-F] [-X] [-P] [-t ['.', '/', 'auto', 'dot', 'fslash']]
                  [-i | -k | -K] [-a] [-A | -Y | -y | -l] [-e] [-x EYAML]
                  [-r PRIVATEKEY] [-u PUBLICKEY]
                  YAML_FILE [YAML_FILE ...]

Returns zero or more YAML Paths indicating where in given YAML/Compatible data
one or more search expressions match. Values, keys, and/or anchors can be
searched. EYAML can be employed to search encrypted values.

positional arguments:
  YAML_FILE             one or more YAML files to search

optional arguments:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit
  -c EXPRESSION, --except EXPRESSION
                        except results matching this search expression; can be
                        set more than once
  -d, --debug           output debugging details
  -v, --verbose         increase output verbosity
  -q, --quiet           suppress all non-result output except errors
  -m, --expand          expand matching parent nodes to list all permissible
                        child leaf nodes (see "reference handling options" for
                        restrictions)
  -t ['.', '/', 'auto', 'dot', 'fslash'], --pathsep ['.', '/', 'auto', 'dot', 'fslash']
                        indicate which YAML Path seperator to use when
                        rendering results; default=dot
  -a, --refnames        also search the names of &anchor and *alias references

required settings:
  -s EXPRESSION, --search EXPRESSION
                        the search expression; can be set more than once

result printing options:
  -L, --values          print the values or elements along with each YAML Path
                        (complex results are emitted as JSON; use --expand to
                        emit only simple values)
  -F, --nofile          omit source file path and name decorators from the
                        output (applies only when searching multiple files)
  -X, --noexpression    omit search expression decorators from the output
  -P, --noyamlpath      omit YAML Paths from the output (useful with --values
                        or to indicate whether a file has any matches without
                        printing them all, perhaps especially with
                        --noexpression)

key name searching options:
  -i, --ignorekeynames  (default) do not search key names
  -k, --keynames        search key names in addition to values and array
                        elements
  -K, --onlykeynames    only search key names (ignore all values and array
                        elements)

reference handling options:
  Indicate how to treat anchor and alias references. An anchor is an
  original, reusable key or value. All aliases become replaced by the
  anchors they reference when YAML data is read. These options specify how
  to handle this duplication of keys and values. Note that the default
  behavior includes all aliased keys but not aliased values.

  -A, --anchorsonly     include only original matching key and value anchors
                        in results, discarding all aliased keys and values
                        (including child nodes)
  -Y, --allowkeyaliases
                        (default) include matching key aliases, permitting
                        search traversal into their child nodes
  -y, --allowvaluealiases
                        include matching value aliases (does not permit search
                        traversal into aliased keys)
  -l, --allowaliases    include all matching key and value aliases

EYAML options:
  Left unset, the EYAML keys will default to your system or user defaults.
  Both keys must be set either here or in your system or user EYAML
  configuration file when using EYAML.

  -e, --decrypt         decrypt EYAML values in order to search them
                        (otherwise, search the encrypted blob)
  -x EYAML, --eyaml EYAML
                        the eyaml binary to use when it isn't on the PATH
  -r PRIVATEKEY, --privatekey PRIVATEKEY
                        EYAML private key
  -u PUBLICKEY, --publickey PUBLICKEY
                        EYAML public key

A search or exception EXPRESSION takes the form of a YAML Path search operator
-- %, $, =, ^, >, <, >=, <=, =~, or ! -- followed by the search term, omitting
the left-hand operand. For more information about YAML Paths, please visit
https://github.com/wwkimball/yamlpath.

• yaml-set

usage: yaml-set [-h] [-V] -g YAML_PATH [-a VALUE | -f FILE | -i | -R LENGTH]
                [-F {bare,boolean,default,dquote,float,folded,int,literal,squote}]
                [-c CHECK] [-s YAML_PATH] [-m] [-b]
                [-t ['.', '/', 'auto', 'dot', 'fslash']] [-e] [-x EYAML]
                [-r PRIVATEKEY] [-u PUBLICKEY] [-d | -v | -q]
                YAML_FILE

Changes one or more values in a YAML file at a specified YAML Path. Matched
values can be checked before they are replaced to mitigate accidental change.
When matching singular results, the value can be archived to another key
before it is replaced. Further, EYAML can be employed to encrypt the new
values and/or decrypt an old value before checking them.

positional arguments:
  YAML_FILE             the YAML file to update

optional arguments:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit
  -F {bare,boolean,default,dquote,float,folded,int,literal,squote}, --format {bare,boolean,default,dquote,float,folded,int,literal,squote}
                        override automatic formatting of the new value
  -c CHECK, --check CHECK
                        check the value before replacing it
  -s YAML_PATH, --saveto YAML_PATH
                        save the old value to YAML_PATH before replacing it;
                        implies --mustexist
  -m, --mustexist       require that the --change YAML_PATH already exist in
                        YAML_FILE
  -b, --backup          save a backup YAML_FILE with an extra .bak file-
                        extension
  -t ['.', '/', 'auto', 'dot', 'fslash'], --pathsep ['.', '/', 'auto', 'dot', 'fslash']
                        indicate which YAML Path seperator to use when
                        rendering results; default=dot
  -d, --debug           output debugging details
  -v, --verbose         increase output verbosity
  -q, --quiet           suppress all output except errors

required settings:
  -g YAML_PATH, --change YAML_PATH
                        YAML Path where the target value is found

input options:
  -a VALUE, --value VALUE
                        set the new value from the command-line instead of
                        STDIN
  -f FILE, --file FILE  read the new value from file (discarding any trailing
                        new-lines)
  -i, --stdin           accept the new value from STDIN (best for sensitive
                        data)
  -R LENGTH, --random LENGTH
                        randomly generate a replacement value of a set length

EYAML options:
  Left unset, the EYAML keys will default to your system or user defaults.
  You do not need to supply a private key unless you enable --check and the
  old value is encrypted.

  -e, --eyamlcrypt      encrypt the new value using EYAML
  -x EYAML, --eyaml EYAML
                        the eyaml binary to use when it isn't on the PATH
  -r PRIVATEKEY, --privatekey PRIVATEKEY
                        EYAML private key
  -u PUBLICKEY, --publickey PUBLICKEY
                        EYAML public key

When no changes are made, no backup is created, even when -b/--backup is
specified. For more information about YAML Paths, please visit
https://github.com/wwkimball/yamlpath.

Libraries

While there are several supporting library files like enumerations, types, and exceptions, the most interesting library files include:

• yamlpath.py -- The core YAML Path parser logic.
• processor.py -- Processes YAMLPath instances to read or write data to YAML/Compatible sources.
• eyamlprocessor.py -- Extends the Processor class to support EYAML data encryption and decryption.

Basic Usage

The files of this project can be used either as command-line tools or as libraries to supplement your own work.

Basic Usage: Command-Line Tools

The command-line tools are self-documented and their documentation is captured above for easy reference. Simply pass --help to them in order to obtain the same detailed documentation.

Please review the comprehensive test_commands_*.py unit tests to explore samples of YAML files and the many ways these tools help get and set their data.

The following are some simple examples of their typical use-cases.

Rotate Your EYAML Keys

If the eyaml command is already on your PATH (if not, be sure to also supply the optional --eyaml or -x argument):

eyaml-rotate-keys \
  --oldprivatekey=~/old-keys/private_key.pkcs7.pem \
  --oldpublickey=~/old-keys/public_key.pkcs7.pem \
  --newprivatekey=~/new-keys/private_key.pkcs7.pem \
  --newpublickey=~/new-keys/public_key.pkcs7.pem \
  my_1st_yaml_file.yaml my_2nd_yaml_file.eyaml ... my_Nth_yaml_file.yaml

You could combine this with find and xargs if your E/YAML file are dispersed through a directory hierarchy, as with Hiera data.

EYAML Compatibility Alert

The maintainers of the hiera-eyaml project have released version 3.x and it is not backward compatible with encryption certificates generated for hiera-eyaml version 2.x. This has nothing to do with YAML Path and is alerted here only as a courtesy to YAML Path users. If you upgrade your installation of hiera-eyaml without first updating your encryption certificates and using a tool like eyaml-rotate-keys (provided here) to re-encrypt your data with the replacement certificates, hiera-eyaml 3.x will fail to decrypt your data! This is not a problem with YAML Path. hiera-eyaml certificate compatibility is well outside the purview of YAML Path and its tools.

Get a YAML Value

At its simplest:

yaml-get \
  --query=see.documentation.above.for.many.samples \
  my_yaml_file.yaml

Search For YAML Paths

Simplest use:

yaml-paths \
  --search=%word \
  /some/directory/*.yaml

Search for multiple expressions and exclude unwanted results:

yaml-paths \
  --search=^another \
  --search=$word \
  --except=%bad \
  /some/directory/*.yaml

Return all leaf nodes under matching parents (most useful when matching against Hash keys and you only want the original leaf nodes beneath them):

yaml-paths \
  --expand \
  --keynames \
  --search==parent_node \
  /some/directory/*.yaml

Change a YAML Value

For a no-frills change to a YAML file with deeply nested Hash structures:

yaml-set \
  --change=see.documentation.above.for.many.samples \
  --value="New Value" \
  my_yaml_file.yaml

To rotate a password, preserving the old password perhaps so your automation can apply the new password to your application(s):

yaml-set \
  --mustexist \
  --change=the.new.password \
  --saveto=the.old.password \
  --value="New Password" \
  my_yaml_file.yaml

For the extremely cautious, you could check the old password before rotating it and save a backup of the original file:

yaml-set \
  --mustexist \
  --change=the.new.password \
  --saveto=the.old.password \
  --check="Old Password" \
  --value="New Password" \
  --backup \
  my_yaml_file.yaml

You can also add EYAML encryption (assuming the eyaml command is on your PATH; if not, you can pass --eyaml to specify its location). In this example, I add the optional --format=folded so that the long EYAML value is broken up into a multi-line value rather than one very long string. This is the preferred format for human legibility as well as EYAML consumers like Puppet. Note that --format has several other settings and applies only to new values.

yaml-set \
  --change=the.new.password \
  --mustexist \
  --saveto=the.old.password \
  --check="Old Password" \
  --value="New Password" \
  --eyamlcrypt \
  --format=folded \
  --backup \
  my_yaml_file.yaml

You can even tell EYAML which keys to use, if not your default system or user keys:

yaml-set \
  --change=the.new.password \
  --mustexist \
  --saveto=the.old.password \
  --check="Old Password" \
  --value="New Password" \
  --eyamlcrypt \
  --format=folded \
  --privatekey=/secret/keys/private_key.pkcs7.pem \
  --publickey=/secret/keys/public_key.pkcs7.pem \
  --backup \
  my_yaml_file.yaml

Note that for even greater security scenarios, you can keep the new value off of your command-line, process list, and command history by swapping out --value for one of --stdin, --file, or even --random LENGTH (use Python's strongest random value generator if you don't need to specify the replacement value in advance).

Basic Usage: Libraries

As for the libraries, they are also heavily documented and the example implementations may perhaps serve as good copy-paste fodder (provided you give credit to the source). That said, here's a general flow/synopsis.

Initialize ruamel.yaml and These Helpers

Your preferences may differ, but I use this setup for round-trip YAML parsing and editing with ruamel.yaml. When you need to process EYAML encrypted data, replace yamlpath.Processor with yamlpath.eyaml.EYAMLProcessor and add error handling for yamlpath.eyaml.EYAMLCommandException.

Note that import yamlpath.patches is entirely optional. I wrote and use it to block ruamel.yaml's Emitter from injecting unnecessary newlines into folded values (it improperly converts every single new-line into two for left-flushed multi-line values, at the time of this writing). Since "block" output EYAML values are left-flushed multi-line folded strings, this fix is necessary when using EYAML features. At least, until ruamel.yaml has its own fix for this issue.

Note also that these examples use ConsolePrinter to handle STDOUT and STDERR messaging. You don't have to. However, some kind of logger must be passed to these libraries so they can write messages somewhere. Your custom message handler or logger must provide the same API as ConsolePrinter; review the header documentation in consoleprinter.py for details. Generally speaking, it would be trivial to write your own custom wrapper for Python's standard logging facilities if you require targets other than STDOUT and STDERR.

import sys

from ruamel.yaml import YAML
from ruamel.yaml.parser import ParserError

import yamlpath.patches
from yamlpath.func import get_yaml_data, get_yaml_editor
from yamlpath.wrappers import ConsolePrinter
from yamlpath import Processor

# Process command-line arguments and initialize the output writer
args = processcli()
log = ConsolePrinter(args)

# Prep the YAML parser and round-trip editor (tweak to your needs)
yaml = get_yaml_editor()

# At this point, you'd load or parse your YAML file, stream, or string.  When
# loading from file, I typically follow this pattern:
yaml_data = get_yaml_data(yaml, log, yaml_file)
if yaml_data is None:
    # There was an issue loading the file; an error message has already been
    # printed.
    exit(1)

# Pass the log writer and parsed YAML data to the YAMLPath processor
processor = Processor(log, yaml_data)

# At this point, the processor is ready to handle YAMLPaths

Searching for YAML Nodes

These libraries use Generators to get nodes from parsed YAML data. Identify which node(s) to get via YAML Path strings. You should also catch yamlpath.exceptions.YAMLPathExceptions unless you prefer Python's native stack traces. When using EYAML, you should also catch yamlpath.eyaml.exceptions.EYAMLCommandExceptions for the same reason. Whether you are working with a single result or many, you should consume the Generator output with a pattern similar to:

from yamlpath import YAMLPath
from yamlpath.exceptions import YAMLPathException

yaml_path = YAMLPath("see.documentation.above.for.many.samples")
try:
    for node_coordinate in processor.get_nodes(yaml_path):
        log.debug("Got {} from '{}'.".format(node_coordinate, yaml_path))
        # Do something with each node_coordinate.node (the actual data)
except YAMLPathException as ex:
    # If merely retrieving data, this exception may be deemed non-critical
    # unless your later code absolutely depends upon a result.
    log.error(ex)

Changing Values

At its simplest, you only need to supply the the YAML Path to one or more nodes to update, and the value to apply to them. Catching yamlpath.exceptions.YAMLPathException is optional but usually preferred over allowing Python to dump the call stack in front of your users. When using EYAML, the same applies to yamlpath.eyaml.exceptions.EYAMLCommandException.

from yamlpath.exceptions import YAMLPathException

try:
    processor.set_value(yaml_path, new_value)
except YAMLPathException as ex:
    log.critical(ex, 119)
except EYAMLCommandException as ex:
    log.critical(ex, 120)