Style guidelines

As mentioned in we use use pylint to check for style violations. Pylint attempts to enforce styles in PEP 8. To see which lint checks we enforce, see the dev_tools/conf/.pylintrc file.

Here we include some extra style guidelines.

Import statements

We follow the import standards of PEP 8, with the following guidance.

In Cirq's main implementation code (not testing code), we prefer importing the full module. This aids in mocking during tests. Thus we prefer

from cirq import ops
qubit = ops.NamedQubit('a')

in contrast to

from cirq.ops import NamedQubit
qubit = NamedQubit('a')

or (the one we would prefer, but doing this causes cyclic dependencies)

import cirq
qubit = cirq.NamedQubit('a')

The one exception to this is for the typing code, where we prefer the direct import

from typing import List

This exception allows typing hints to be more compact.

In tests, however, we prefer that we use Cirq as you would use cirq externally. For code that is in the Cirq core framework this is

import cirq
qubit = cirq.NamedQubit('a')

For Cirq code that is outside of the core and does not appear at the cirq module level, for example work in contrib, one should use the highest level possible for test code

import cirq
from cirq import contrib

Of course, if this import style fundamentally cannot be used, do not let this block submitting a pull request for the code as we will definitely grant exceptions.

Typing based import cycles

An import cycle is where modules need to import each other (perhaps indirectly). Sometimes in order to add a type annotation you have to add an import which causes a cycle. To avoid this we use the TYPE_CHECKING constant provided by typing:

from typing import TYPE_CHECKING
    # pylint: disable=unused-import
    import module.that.causes.cycle

Note that if you do this you will need to use the string version of the type,

def my_func() -> 'module.that.causes.cycle.MyClass':


Using consistent wording across Cirq is important for lowering users cognitive load. For rule governing naming, see the nomenclature guidelines.