为什么只导入类时运行类定义之外的代码？

Question

期望
我想从另一个文件中导入一个 python 类而不导入文件的其余部分。

问题
当我使用from Exercise_7 import Vehicle（Vehicle 是类）导入类，然后运行我的代码时，Exercise_7.py 中类定义之外的行似乎也在运行。

代码

Exercise_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))

Exercise_8.py（运行时运行不需要的代码的文件）：

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))

Exercise_8.py 运行时，输出如下：

['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

Answer 1

模块是可执行代码块，def 和 class 语句也是如此。它们仅在其代码的整个上下文中被明确定义。因此，不可能只加载模块的一部分。

运行from Exercise_7 import Vehicle 仍然需要加载并执行整个模块Exercise_7 到解释器中，即使它只是导入和绑定名称Vehicle 到文件。

---

不应该在常规导入期间运行的代码应该放在 __main__ guard 中：

if __name__ == "__main__":
    vehicle1 = Vehicle("Car", (3,5), "red", False)
    vehicle1.updateVelocity((2,6))
    vehicle1.allAttributes()
    print(vehicle1.emissions(100))

简单地说，一个模块只有在直接执行时才是__main__ - 例如。与 python -m Exercise_7 一样——但在被另一个模块导入时不会。

Answer 2

这就是解释语言的工作方式（如 python、javascript ...）任何存在于函数或类定义之外的代码都将由解释器（在您的情况下为 python 解释器）执行