How can I do string concatenation, or string replacement in YAML?

I have this:

user_dir: /home/user
user_pics: /home/user/pics

How could I use the user_dir for user_pics? If I have to specify other properties like this, it would not be very DRY.

You can use a repeated node, like this:

user_dir: &user_home /home/user
user_pics: *user_home

I don't think you can concatenate though, so this wouldn't work:

user_dir: &user_home /home/user
user_pics: *user_home/pics

It's surprising, since the purpose of YAML anchors & references is to factor duplication out of YAML data files, that there isn't a built-in way to concatenate strings using references. Your use case of building up a path name from parts is a good example -- there must be many such uses.

Fortunately there's a simple way to add string concatenation to YAML via custom tags in Python.

import yaml

## define custom tag handler
def join(loader, node):
    seq = loader.construct_sequence(node)
    return ''.join([str(i) for i in seq])

## register the tag handler
yaml.add_constructor('!join', join)

## using your sample data
yaml.load("""
user_dir: &DIR /home/user
user_pics: !join [*DIR, /pics]
""")

Which results in:

{'user_dir': '/home/user', 'user_pics': '/home/user/pics'}

You can add more items to the array, like " " or "-", if the strings should be delimited.

If you are using python with PyYaml, joining strings is possible within the YAML file. Unfortunately this is only a python solution, not a universal one:

with os.path.join:

user_dir: &home /home/user
user_pics: !!python/object/apply:os.path.join [*home, pics]

with string.join (for completeness sake - this method has the flexibility to be used for multiple forms of string joining:

user_dir: &home /home/user
user_pics: !!python/object/apply:string.join [[*home, pics], /]

I would use an array, then join the string together with the current OS Separator Symbol

like this:

default: &default_path "you should not use paths in config"
pictures:
  - *default_path
  - pics

Seems to me that YAML itself does not define way to do this.

Good news are that YAML consumer might be able to understand variables.
What will use Your YAML?

string.join() won't work in Python3, but you can define a !join like this:

import functools
import yaml

class StringConcatinator(yaml.YAMLObject):
    yaml_loader = yaml.SafeLoader
    yaml_tag = '!join'
    @classmethod
    def from_yaml(cls, loader, node):
        return functools.reduce(lambda a, b: a.value + b.value, node.value)

c=yaml.safe_load('''
user_dir: &user_dir /home/user
user_pics: !join [*user_dir, /pics]''')
print(c)

As of August 2019:

To make Chris' solution work, you actually need to add Loader=yaml.Loader to yaml.load(). Eventually, the code would look like this:

import yaml

## define custom tag handler
def join(loader, node):
    seq = loader.construct_sequence(node)
    return ''.join([str(i) for i in seq])

## register the tag handler
yaml.add_constructor('!join', join)

## using your sample data
yaml.load("""
user_dir: &DIR /home/user
user_pics: !join [*DIR, /pics]
""", Loader=yaml.Loader)

See this GitHub issue for further discussion.