在一段時間內在Python中僅執行一次if語句？

Question

創建一個從arduino距離傳感器接收數據的python腳本。 我每納秒都會收到一個值。 每當此值大於50時，我都想警告用戶。 （最終將通過通知程序執行此操作，但現在我只是打印警告）。 我有以下內容：

while 1:                                                   # Keep receiving data
    data = ser.readline()                                  # Getting data from arduino
    value = [int(s) for s in data.split() if s.isdigit()]  # Converting the Arduino ouput to only get  the integer value
    if value:                                              # Avoid empty values
        distance = value[0]                                # List to int
            if distance > 50:                              # If bigger than 50 cm, warn user.
                warn_user()                 

我只想在30秒內執行一次warn_user（）函數，此后，僅當值降至50以下且THEN> 50再次時，if語句才不再觸發。 我嘗試使用True / False語句，但計時器卻睡着了，但這沒有用。 有小費嗎？ 謝謝。

Answer 1

您只需要添加更多邏輯條件即可控制程序的流程。 這樣的事情會起作用：

from time import time
warning_enabled = True
time_of_warning = 0

while 1:
    data = ser.readline()
    value = [int(s) for s in data.split() if s.isdigit()]
    if value:
        distance = value[0]
            if distance > 50 and warning_enabled:
                warn_user()
                warning_enabled = False
                time_of_warning = time()
            if time() - time_of_warning > 30 and not warning_enabled and distance < 50:
                warning_enabled = True

這是因為它會跟蹤上一次觸發警告的時間，並使用warning_enable標志使第二次（ if僅在可能的if下）觸發。

干杯

Answer 2

您只需跟蹤要實現目標的所需內容：上次警告的時間戳以及距離是否低於您要跟蹤的值。

import time

distance_was_safe = True  # tracks if distance went below specified range
last_warned = 0           # tracks timestamp since last warning

safe_distance = 50  # 50 cm
warn_interval = 30  # 30 sec, warn no more than once every interval


while True:
    # Get data from Arduino.
    data = ser.readline()                                  

    # Convert the Arduino output to only get the integer values.
    value = [int(s) for s in data.split() if s.isdigit()]

    # Avoid empty output.
    if value:                                              
        # Get the first integer.
        distance = value[0]

            # If greater than safe zone, potentially warn the user.
            if distance > safe_distance:

                # Warn the user if distance was below range,
                # and it has been enough time since the last warning.
                if distance_was_safe and time.time() - last_warned > warn_interval:
                    distance_was_safe = False
                    last_warned = time.time()
                    warn_user()

            else:
                # Distance was less than warning distance, reset the flag.
                distance_was_safe = True