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---
title: Empty key validation
---
Sometimes you may wish to not specify a value or specify
that it does not exist.
Using StrictYAML you can accept this as a valid value and
have it parsed to one of three things - None, {} (empty dict),
or [] (empty list).
Example yaml_snippet:
```yaml
a:
```
```python
from strictyaml import Map, Str, Enum, EmptyNone, EmptyDict, EmptyList, NullNone, load, as_document
from ensure import Ensure
```
EmptyNone with empty value:
```python
Ensure(load(yaml_snippet, Map({"a": EmptyNone() | Enum(["A", "B",])}))).equals({"a": None})
```
EmptyDict:
```python
Ensure(load(yaml_snippet, Map({"a": EmptyDict() | Enum(["A", "B",])}))).equals({"a": {}})
```
EmptyList:
```python
Ensure(load(yaml_snippet, Map({"a": EmptyList() | Enum(["A", "B",])}))).equals({"a": []})
```
NullNone:
```python
Ensure(load("a: null", Map({"a": NullNone() | Enum(["A", "B",])}))).equals({"a": None})
```
EmptyNone no empty value:
```yaml
a: A
```
```python
Ensure(load(yaml_snippet, Map({"a": EmptyNone() | Enum(["A", "B",])}))).equals({"a": "A"})
```
Combine Str with EmptyNone and Str is evaluated first:
```python
Ensure(load(yaml_snippet, Map({"a": Str() | EmptyNone()}))).equals({"a": ""})
```
Combine EmptyNone with Str and Str is evaluated last:
```python
Ensure(load(yaml_snippet, Map({"a": EmptyNone() | Str()}))).equals({"a": None})
```
Non-empty value:
```yaml
a: C
```
```python
load(yaml_snippet, Map({"a": Enum(["A", "B",]) | EmptyNone()}))
```
```python
strictyaml.exceptions.YAMLValidationError:
when expecting an empty value
found arbitrary text
in "<unicode string>", line 1, column 1:
a: C
^ (line: 1)
```
Serialize empty dict:
```python
print(as_document({"a": {}}, Map({"a": EmptyDict() | Str()})).as_yaml())
```
```yaml
a:
```
Serialize empty list:
```python
print(as_document({"a": []}, Map({"a": EmptyList() | Str()})).as_yaml())
```
```yaml
a:
```
Serialize None:
```python
print(as_document({"a": None}, Map({"a": EmptyNone() | Str()})).as_yaml())
```
```yaml
a:
```
!!! note "Executable specification"
Documentation automatically generated from
<a href="https://github.com/crdoconnor/strictyaml/blob/master/hitch/story/empty.story">empty.story
storytests.
|
