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---
title: Validating optional keys in mappings (Map)
---
Not every key in a YAML mapping will be required. If
you use the "Optional('key')" validator with YAML,
you can signal that a key/value pair is not required.
```python
from strictyaml import Map, Int, Str, Bool, Optional, load
from ensure import Ensure
schema = Map({"a": Int(), Optional("b"): Bool(), })
```
Valid example 1:
```yaml
a: 1
b: yes
```
```python
Ensure(load(yaml_snippet, schema)).equals({"a": 1, "b": True})
```
Valid example 2:
```yaml
a: 1
b: no
```
```python
Ensure(load(yaml_snippet, schema)).equals({"a": 1, "b": False})
```
Valid example missing key:
```yaml
a: 1
```
```python
Ensure(load(yaml_snippet, schema)).equals({"a": 1})
```
Invalid 1:
```yaml
a: 1
b: 2
```
```python
load(yaml_snippet, schema)
```
```python
strictyaml.exceptions.YAMLValidationError:
when expecting a boolean value (one of "yes", "true", "on", "1", "y", "no", "false", "off", "0", "n")
found an arbitrary integer
in "<unicode string>", line 2, column 1:
b: '2'
^ (line: 2)
```
Invalid 2:
```yaml
a: 1
b: yes
c: 3
```
```python
load(yaml_snippet, schema)
```
```python
strictyaml.exceptions.YAMLValidationError:
while parsing a mapping
unexpected key not in schema 'c'
in "<unicode string>", line 3, column 1:
c: '3'
^ (line: 3)
```
!!! note "Executable specification"
Documentation automatically generated from
<a href="https://github.com/crdoconnor/strictyaml/blob/master/hitch/story/optional.story">optional.story
storytests.
|
