UnboundLocalError：分配前引用了局部变量'x1'

Question

import numpy as np
from PIL import ImageGrab
import cv2
import time
import pyautogui
import matplotlib.pyplot as plt


def make_coords(img,line_param):
    slope,intercept=line_param

    y1 = img.shape[0]
    y2 = int((y1*(3/5)))
    x1 = int((y1-intercept)/slope)
    x2 = int((y2-intercept)/slope)
    try:
        return np.array((x1,y1,x2,y2))    #HERE IS WHERE THE PROBLEM HAPPENS
    except UnboundLocalError:
        pass


def avg_slope(img,lines):
    left_fit =[]
    right_fit=[]
    if lines is not None:
        for line in lines:
            x1,y1,x2,y2=line.reshape(4)
            parameters = np.polyfit((x1,x2),(y1,y2),1)
            try:
                slope = parameters[0]
            except TypeError:
                slope = 0
            try:
                intercept = parameters[1]
            except TypeError:
                intercept = 0
            if slope <0:
                left_fit.append((slope,intercept))
            else:
                right_fit.append((slope,intercept))
    if left_fit:
        left_fit_avg=np.average(left_fit,axis=0)
        left_line=make_coords(img,left_fit_avg)
    if right_fit:
        right_fit_avg=np.average(right_fit,axis=0)
        right_line=make_coords(img,right_fit_avg)

    return np.array((x1,y1,x2,y2))

def draw_lines(img, lines):
    try:
        for line in lines:
            if line is not None:
                coords = line[0]
                cv2.line(img, (coords[0],coords[1]), (coords[2],coords[3]), [255,0,0], 3)
    except:
        pass


def roi(img):
    vertices = np.array([[10,500],[10,300], [300,200], [500,200], [800,300], [800,500]], np.int32)
    mask = np.zeros_like(img)
    cv2.fillPoly(mask, [vertices], 255)
    masked = cv2.bitwise_and(img, mask)
    return masked

def process_img(image):
    original_image = image
    # convert to gray
    processed_img = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
    # edge detection
    processed_img = cv2.GaussianBlur(processed_img,(5,5),0) #new
    processed_img =  cv2.Canny(processed_img, threshold1 = 50, threshold2=150) #new
    # processed_img =  cv2.Canny(processed_img, threshold1 = 200, threshold2=300)
    lines = cv2.HoughLinesP(processed_img, 1, np.pi/180, 180, np.array([]), minLineLength=15,maxLineGap=5)
    avg_lines = avg_slope(processed_img,lines)

    draw_lines(process_img,avg_lines)
    processed_img = roi(processed_img)
    return processed_img

def main():

    last_time = time.time()
    while True:

        screen =  np.array(ImageGrab.grab(bbox=(0,40,800,640)))
        if screen is not None:
            new_screen = process_img(screen)
            print('Frame took {} seconds'.format(time.time()-last_time))


            cv2.imshow('window', new_screen)
        else:
            pass
        last_time = time.time()

        # plt.imshow(new_screen)
        #cv2.imshow('window',cv2.cvtColor(screen, cv2.COLOR_BGR2RGB))
        # cv2.waitKey(0)
        if cv2.waitKey(25) & 0xFF == ord('q'):
            cv2.destroyAllWindows()

            break
main()

终端显示：

avg_lines = avg_slope(processed_img,lines)
Frame took 0.12310576438903809 seconds
Traceback (most recent call last):
  File "c:/Users/Nicole/Documents/Python Scripts/matetest.py", line 107, in <module>
    main()
  File "c:/Users/Nicole/Documents/Python Scripts/matetest.py", line 91, in main 91, 

in main
        new_screen = process_img(screen)                                78, in process_img
      File "c:/Users/Nicole/Documents/Python Scripts/matetest.py", line 78, in process_img                                                 50, in avg_slope
        avg_lines = avg_slope(processed_img,lines)
      File "c:/Users/Nicole/Documents/Python Scripts/matetest.py", line 50, in avg_slope
        return np.array((x1,y1,x2,y2))
    UnboundLocalError: local variable 'x1' referenced before assignment

...即使我在做...

try:
        return np.array((x1,y1,x2,y2))
    except UnboundLocalError:
        pass

Answer 1

您所说的错误实际上并没有发生。通过查看回溯，您可以看到函数avg_slope中发生了错误。

可能是因为您使用return np.array((x1,y1,x2,y2))，而在该函数中仅在if语句中声明了这些值。如果将跳过if块（当lines为None时），则该函数中未声明x1，x2，y1和y2。换句话说：可能是这些函数在函数内部永远不存在，因此您不能根据它们返回任何内容。解释器阻止您执行此操作。

通过仔细阅读错误消息，您可以学到很多东西。概括地说，分配前引用的局部变量简而言之。

Answer 2

您的问题在这里：

def avg_slope(img,lines):
    left_fit =[]
    right_fit=[]
    if lines is not None:
        for line in lines:
            x1,y1,x2,y2=line.reshape(4)

如果lines为“假”（空或None，则您永远不会分配给x1。