# Constraints

We can add various constraints to parameters, such as "greater than" or "not equal to", which will be matched against the corresponding specified value. In case a constraint is infringed the framework will raise a `ValueError`.

```python
from hanna import Configurable, Integer, String
from pyhocon import ConfigTree

class Movie(Configurable):
    year = Integer() >= 2000  # Constrain to recent movies only.

movie = Movie(ConfigTree(year=1977))  # raises ValueError: "year" Too small (1977 < 2000)
movie = Movie(ConfigTree(year=2018))  # Fine.
```

Constraints can be declared by using any of the comparison operators (`<, <=, >, >=, !=, ==`).

{% hint style="info" %}
Constraints are checked for *after* all [transformations](https://dominik1123.gitbook.io/hanna/transformations) have been applied.
{% endhint %}

### Constraining meta characteristics

Constraints can as well refer to meta characteristics such as `len` or `abs`.

```python
class Movie(Configurable):
    title = String().len >= 10
    
movie = Movie(ConfigTree(title='Star Wars'))  # raises ValueError: "title" Too short (9 < 10)
movie = Movie(ConfigTree(title='Star Wars: A New Hope'))  # Fine.
```

### Custom constraints

Finally we can add constraints manually by invoking the `constrain` method. This method accepts any callable with the convention that the callable shall return `True` if the constraint holds and `False` otherwise.

```python
class Movie(Configurable):
    title = String().constrain(lambda x: len(x.split()) >= 5)  # Only titles with at least 5 words.
    
movie = Movie(ConfigTree(title='Star Wars'))  # raises ValueError: "title"
movie = Movie(ConfigTree(title='Star Wars: A New Hope'))  # Fine.
```

#### Custom error messages

The error message in the previous example is not really verbose since it only contains the name of the parameter which caused the exception. However we can provide a custom error message when declaring a constraint as an additional argument:

```python
class Movie(Configurable):
    title = String().constrain(lambda x: len(x.split()) >= 5,
                               'Not enough words')

movie = Movie(ConfigTree(title='Star Wars'))  # raises ValueError: "title" Not enough words
```

Within the error message we can even refer to the underlying value which will then be displayed in the message:

```python
class Movie(Configurable):
    title = String().constrain(lambda x: len(x.split()) >= 5,
                               'Not enough words ({value} < 5)')

movie = Movie(ConfigTree(title='Star Wars'))  # raises ValueError: "title" Not enough words (2 < 5)
```
