期望
我想從另一個文件導入 python 類而不導入文件的其餘部分。
問題
當我使用from Exercise_7 import Vehicle（Vehicle 是類）導入類，然後運行我的代碼時，似乎來自 Exercise_7.py 的類定義之外的行也運行。
代碼
練習_7.py：
import numpy as np
class Vehicle:
def __init__(self, typeOfVehicle, velocity, colour, electric):
self.typeOfVehicle = str(typeOfVehicle)
self.velocity = tuple(velocity)
self.velocity = np.array(self.velocity)
self.colour = str(colour)
self.electric = bool(electric)
def updateVelocity(self, velocityTuple):
self.velocity = np.array(velocityTuple)
def allAttributes(self):
attributes = [self.typeOfVehicle, self.velocity, self.colour, self.electric]
print(attributes)
def emissions(self, distance):
#electric vehicles
if self.electric == True:
if self.typeOfVehicle == "Car":
emission = 1*distance
elif self.typeOfVehicle == "Truck":
emission = 5*distance
else: #vehicle is not electric
if self.typeOfVehicle == "Car":
emission = 10*distance
elif self.typeOfVehicle == "Truck":
emission = 50*distance
return emission
#---TEST CODE THAT IS BEING RUN (when I don't want it to)---
vehicle1 = Vehicle("Car", (3,5), "red", False)
vehicle1.updateVelocity((2,6))
vehicle1.allAttributes()
print(vehicle1.emissions(100))
練習_8.py（運行時運行不需要的代碼的文件）：
from Exercise_7 import Vehicle
import numpy as np
import random as r
def newVehicles(type, electric1):
colours = ["red", "blue", "green", "black", "grey"]
for i in range(0,50):
velocity = (r.uniform(1,70), r.uniform(1,70))
newVehicle = Vehicle(type, velocity, colours[r.randint(0,4)], electric1)
vehicles.append(newVehicle)
def createVehicles():
newVehicles("Car", True)
newVehicles("Car", False)
newVehicles("Truck", True)
newVehicles("Truck", False)
return vehicles
def calculateEmissions(vehicles, time):
electricCarsEmissions = 0
nonelectricCarsEmissions = 0
electricTrucksEmissions = 0
nonelectricTrucksEmissions = 0
for vehicle in vehicles:
speed = (vehicle.velocity[0]**2 + vehicle.velocity[1]**2)**1/2
distanceTravelled = speed*time
vehicle.distance = distanceTravelled
emission = vehicle.emissions(distanceTravelled)
if vehicle.typeOfVehicle == "Car" and vehicle.electric == True:
electricCarsEmissions += emission
elif vehicle.typeOfVehicle == "Car" and vehicle.electric == False:
nonelectricCarsEmissions += emission
elif vehicle.typeOfVehicle == "Truck" and vehicle.electric == True:
electricTrucksEmissions += emission
elif vehicle.typeOfVehicle == "Truck" and vehicle.electric == False:
nonelectricTrucksEmissions += emission
return electricCarsEmissions, nonelectricCarsEmissions, electricTrucksEmissions, nonelectricTrucksEmissions
vehicles = []
listOfVehicles = createVehicles()
print(calculateEmissions(listOfVehicles, 20))
當運行 Exercise_8.py 時，它輸出以下內容：
['Car', array([2, 6]), 'red', False] #output from Exercise_7.py code
1000 #output from Exercise_7.py code
(1836161.109986278, 16586342.282856379, 7901210.428254995, 74406974.7703749) #desired output
模塊是可執行代碼塊，def和class語句一樣。它們僅在其整個代碼上下文中得到明確定義。因此，不可能只加載模塊的一部分。
運行from Exercise_7 import Vehicle仍然需要將整個模塊加載並執行Exercise_7到解釋器中，即使它只是將名稱導入並綁定Vehicle到文件中。
不應該在常規導入期間運行的代碼應該在一個__main__守衛中：
if __name__ == "__main__":
vehicle1 = Vehicle("Car", (3,5), "red", False)
vehicle1.updateVelocity((2,6))
vehicle1.allAttributes()
print(vehicle1.emissions(100))
鬆散地說，模塊__main__僅在直接執行時（例如作為）python -m Exercise_7，而不是在由另一個模塊導入時。
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