in practice

in reality (used to refer to what actually happens as opposed to what is meant or believed to happen)

实际上（用来指与预期或据信已发生的事相反的实际情况）

in theory this method is ideal—in practice it is unrealistic.

理论上这种方法很理想，——但实际上却不现实。

out of practice

not currently proficient in a particular activity or skill though not having exercised or performed it for some time

不熟练，生疏

he was out of practice at interrogation.

他对审讯不太熟练。

practice makes perfect

used to convey that regular exercise of an activity or skill is the way to become proficient in it, especially when encouraging someone to persist in it

（尤用于鼓励）熟能生巧