---
title: order_terms (latest version)
description: API reference for qiskit_fermions.operators.terms.order_terms in the latest version of qiskit-fermions
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit-fermions/operators-terms-order-terms
---

# order\_terms

`order_terms(op, key, *, reverse=False)`

Returns a copy of an operator with its terms reordered by a user-provided key.

Unlike [`canonical_order()`](/docs/api/qiskit-fermions/operators-terms-canonical-order "qiskit_fermions.operators.terms.canonical_order"), which imposes a single fixed, coefficient-independent order, this function lets you sort the terms by any criterion you like: the `key` function is applied to each term exactly as [`sorted()`](https://docs.python.org/3/library/functions.html#sorted) would, and the terms are rebuilt into a new operator of the same type in the resulting order. This is useful whenever the *order* of the terms matters for a downstream computation (e.g. randomized product-formula sampling), since it gives you a deterministic, reproducible order independent of how the operator happened to be assembled.

The terms themselves are left untouched – this only reorders them, it does not simplify or normal-order the operator. This works for any of the built-in operator types implementing the [`OperatorTrait`](/docs/api/qiskit-fermions/operators-operator-trait "qiskit_fermions.operators.OperatorTrait") protocol; the returned operator is of the same type as the input.

> **Note**
>
> The `key` receives each term as the same tuple that the operator’s [`iter_terms()`](/docs/api/qiskit-fermions/operators-operator-trait#iter_terms "qiskit_fermions.operators.OperatorTrait.iter_terms") yields – or, when the operator tracks groups (see [`groups`](/docs/api/qiskit-fermions/operators-operator-trait#groups "qiskit_fermions.operators.OperatorTrait.groups")), the longer tuple from [`iter_terms_with_groups()`](/docs/api/qiskit-fermions/operators-operator-trait#iter_terms_with_groups "qiskit_fermions.operators.OperatorTrait.iter_terms_with_groups"), whose trailing element is the term’s group index. The exact layout of the term itself depends on the operator type. Group indices are preserved – each term carries its group index along as it moves – and if the input tracks no groups, neither does the result.

```pycon
>>> from qiskit_fermions.operators import FermionOperator
>>> from qiskit_fermions.operators.terms.ordering import order_terms
>>> op = FermionOperator.from_dict(
...     {
...         ((True, 1), (False, 0)): 1.0,  # a†_1 a_0, |coeff| = 1
...         ((True, 0), (False, 1)): 3.0,  # a†_0 a_1, |coeff| = 3
...     }
... )
>>> ordered = order_terms(op, key=lambda term: abs(term[1]), reverse=True)
>>> list(ordered.iter_terms())
[([(True, 0), (False, 1)], (3+0j)), ([(True, 1), (False, 0)], (1+0j))]
```

**Parameters**

- **op** ([*OperatorTrait*](/docs/api/qiskit-fermions/operators-operator-trait "qiskit_fermions.operators.operator_trait.OperatorTrait")) – the operator whose terms to reorder.
- **key** ([*Callable*](https://docs.python.org/3/library/collections.abc.html#collections.abc.Callable)*\[\[*[*tuple*](https://docs.python.org/3/library/stdtypes.html#tuple)*],* [*Any*](https://docs.python.org/3/library/typing.html#typing.Any)*]*) – a function of a single term, used exactly as the `key` argument of [`sorted()`](https://docs.python.org/3/library/functions.html#sorted). It receives each term as the tuple described in the note above.
- **reverse** ([*bool*](https://docs.python.org/3/library/functions.html#bool)) – if `True`, the terms are sorted in descending order of `key`.

**Returns**

A new operator of the same type with its terms in the requested order.

**Return type**

[*OperatorTrait*](/docs/api/qiskit-fermions/operators-operator-trait "qiskit_fermions.operators.operator_trait.OperatorTrait")