One flower does not make a spring; one swallow does not make a summer.

鲜花一朵还不是春天，孤燕一只不成夏天。

youdao

One swallow does not make a summer (saying) a single fortunate or satisfactory incident, example, etc does not mean that all the others will be AS good.

谚一燕不成夏仅有的一次幸运或好事等并不意味着一切都好。

youdao

One swallow does not a summer make, of course.

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"For the high street, one swallow does not make a summer, but these results might hint at the green shoots of recovery, or at least some stabilisation in the current environment, " Ms Wehrle added.

BBC: Shopper