> ## Documentation Index
> Fetch the complete documentation index at: https://docs.pixeltable.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Expr
> 
# class pixeltable.exprs.Expr
A Pixeltable expression.
All Pixeltable expressions, including [column references](./columnref) (such as `t.col`),
UDF calls (`t.my_string.lower()`), and compound expressions (`t.col + 5`) are instances of this class.
Not thread-safe: Expr and subclasses contain execution state and are never thread-safe.
## method astype()
```python Signature theme={"theme":{"light":"light-plus","dark":"dark-plus"}}
astype(new_type: ts.ColumnType | type | _AnnotatedAlias) -> exprs.TypeCast
```
Return a new expression that casts this expression to a different type.
This represents a type *cast*, not a type *coercion*, so it will not mutate the underlying data; it simply
changes the static type given by the expression.
**Parameters:**
* **`new_type`** (`ts.ColumnType | type | _AnnotatedAlias`): The type to cast to.
**Examples:**
Given an existing column `t.json_col` of type `pxt.Json`, cast that column to type `pxt.String`:
```python theme={"theme":{"light":"light-plus","dark":"dark-plus"}}
t.json_col.astype(pxt.String)
```
This will assume that all values in `t.json_col` are *actually* strings, and it will result in an error
at runtime if there are values in `t.json_col` that are not strings. It will *not* convert those values to
a string representation. (For that, use the \[`dumps()`]\[pixeltable.functions.json.dumps] UDF instead.)
## method isin()
```python Signature theme={"theme":{"light":"light-plus","dark":"dark-plus"}}
isin(value_set: Any) -> exprs.InPredicate
```
Return a new, boolean-valued expression that is `True` whenever this expression is in `value_set`.
**Parameters:**
* **`value_set`** (`Any`): Either another expression that evaluates to a set of values, or a constant collection of
values. If the latter, can be any `Iterable`.
**Examples:**
These examples assume that `t` is a table with a column `int_col` of type `pxt.Int`, and another column
`list_col` of type `pxt.Json`, containing lists of integers.
Select all rows where `int_col` is in the constant set `{1, 3, 22}`:
```python theme={"theme":{"light":"light-plus","dark":"dark-plus"}}
t.where(t.int_col.isin({1, 3, 22})).select()
```
Select all rows where `int_col` is in the set of values in that row's `list_col`:
```python theme={"theme":{"light":"light-plus","dark":"dark-plus"}}
t.where(t.int_col.isin(t.list_col)).select()
```