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---
title: Sequence/list validator (Seq)
---
Sequences in YAML are denoted by a series of dashes ('-')
and parsed as a list in python.
Validating sequences of a particular type can be done with
the Seq validator, specifying the type.
See also [UniqueSeq](../sequences-of-unique-items) and
[FixedSeq](../fixed-length-sequences) for other types of sequence
validation.
Example yaml_snippet:
```yaml
- 1
- 2
- 3
```
```python
from strictyaml import Seq, Str, Int, load
from ensure import Ensure
```
Valid Parsed:
```python
Ensure(load(yaml_snippet, Seq(Str()))).equals(["1", "2", "3", ])
```
Is sequence:
```python
assert load(yaml_snippet, Seq(Str())).is_sequence()
```
Iterator:
```python
assert [x for x in load(yaml_snippet, Seq(Str()))] == ["1", "2", "3"]
```
Lists of lists:
```yaml
-
- a
- b
- c
-
- d
- e
- f
```
```python
assert load(yaml_snippet, Seq(Seq(Str()))) == [["a", "b", "c"], ["d", "e", "f"]]
```
.text is nonsensical:
```yaml
- â
- 2
- 3
```
```python
load(yaml_snippet, Seq(Str())).text
```
```python
builtins.TypeError:YAML(['â', '2', '3']) is a sequence, has no text value.:
```
Invalid mapping instead:
```yaml
a: 1
b: 2
c: 3
```
```python
load(yaml_snippet, Seq(Str()))
```
```python
strictyaml.exceptions.YAMLValidationError:
when expecting a sequence
in "<unicode string>", line 1, column 1:
a: '1'
^ (line: 1)
found a mapping
in "<unicode string>", line 3, column 1:
c: '3'
^ (line: 3)
```
Invalid nested structure instead:
```yaml
- 2
- 3
- a:
- 1
- 2
```
```python
load(yaml_snippet, Seq(Str()))
```
```python
strictyaml.exceptions.YAMLValidationError:
when expecting a str
in "<unicode string>", line 3, column 1:
- a:
^ (line: 3)
found a mapping
in "<unicode string>", line 5, column 1:
- '2'
^ (line: 5)
```
Invalid item in sequence:
```yaml
- 1.1
- 1.2
```
```python
load(yaml_snippet, Seq(Int()))
```
```python
strictyaml.exceptions.YAMLValidationError:
when expecting an integer
found an arbitrary number
in "<unicode string>", line 1, column 1:
- '1.1'
^ (line: 1)
```
One invalid item in sequence:
```yaml
- 1
- 2
- 3.4
```
```python
load(yaml_snippet, Seq(Int()))
```
```python
strictyaml.exceptions.YAMLValidationError:
when expecting an integer
found an arbitrary number
in "<unicode string>", line 3, column 1:
- '3.4'
^ (line: 3)
```
!!! note "Executable specification"
Documentation automatically generated from
<a href="https://github.com/crdoconnor/strictyaml/blob/master/hitch/story/sequence.story">sequence.story
storytests.
|
