为什么这个随机Next（）抛出异常？(Why does this random Next() throw an exception?)

 我相信这会有效，你在printer（）函数中不需要return语句： 
 
import sys
subjects = ["History", "Science"]
topics_science = ["Light", "X-mas Exam Review"]
topics_history = ["Italian Renaissance"]
science_xmasreview = ["Q. What do we use to catogorize a habitat?",
                      "A. Damp or Dry, Hot or Cold, Windy or Calm, Dim or Bright.",
                      "Q. What is something that only eats plants?",
                      "A. Herbivore",
                      "Q. What are the properties of oxygen?"]

science_light = [
    "Q. What is an object the gives out light?",
    "A. Light source",
    "Q. What is the speed of light?",
    "A. 300 million meters per second.",
    "Q. What does transparent mean?",
    "A. Light can pass completely through a transparent material."]

history_renaissance = [
    "Q. What did Lorenzo do differently from Cosimo?",
    "A. Lorenzo made no effort to conceal his power",
    "Q. Why did the Pope want Lorenzo dead?",
    "A. Because the Pope wanted more power and saw Lorenzo as a threat.",
    "Q. Who did the Pazzi plot with to kill Lorenzo?",
    "A. Pope Sixtus IV"]

def printer(list):
    n = 0
    l = len(list)
    print
    while n < l:
        print list[n]  # <-- Changed this from a return to a print statement
        newline()
        n += 1
    while n == l:
        n += 1

def qanda(x):
    print
    print "Enter 1 for just questions"
    print "Enter 2 for just answers"
    print "Enter 3 for both"
    qa = raw_input("Enter number: ")
    qa = str(qa)
    if qa == '1':
        printer(x[::2])
    elif qa == '2':
        printer(x[1::2])
    elif qa == '3':
        printer(x)
    else:
        print "Not recognized"

def newline():
    raw_input()
    print



def subjectchoice():
    if subject == "1":
        print
        history()
    elif subject == "2":
        print
        science()
    else:
        print 'Not recognized.'
def science():
    print topics_science
    print "Enter 1 for Light"
    print "Enter 2 for X-mas Exam Review"
    topicchoice = raw_input("Enter number: ")
    topicchoice = str(topicchoice)
    if topicchoice == "1":
        qanda(science_light)
    elif topicchoice == "2":
        qanda(science_xmasreview)
    else:
        print "Not recognized"
        sys.exit
def history():
    print topics_history
    print "Enter 1 for Italian Renaissance"
    topicchoice = raw_input("Enter number: ")
    topicchoice = str(topicchoice)
    if topicchoice == "1":
        return qanda(history_renaissance)
    else:
        print "Not recognized"
        sys.exit()
print subjects
print "Enter 1 for History"
print "Enter 2 for Science"
subject = raw_input("Enter number: ")
subject = str(subject)
subjectchoice()

I believe this will work, you didn't need the return statement in your printer() function:
 
import sys
subjects = ["History", "Science"]
topics_science = ["Light", "X-mas Exam Review"]
topics_history = ["Italian Renaissance"]
science_xmasreview = ["Q. What do we use to catogorize a habitat?",
                      "A. Damp or Dry, Hot or Cold, Windy or Calm, Dim or Bright.",
                      "Q. What is something that only eats plants?",
                      "A. Herbivore",
                      "Q. What are the properties of oxygen?"]

science_light = [
    "Q. What is an object the gives out light?",
    "A. Light source",
    "Q. What is the speed of light?",
    "A. 300 million meters per second.",
    "Q. What does transparent mean?",
    "A. Light can pass completely through a transparent material."]

history_renaissance = [
    "Q. What did Lorenzo do differently from Cosimo?",
    "A. Lorenzo made no effort to conceal his power",
    "Q. Why did the Pope want Lorenzo dead?",
    "A. Because the Pope wanted more power and saw Lorenzo as a threat.",
    "Q. Who did the Pazzi plot with to kill Lorenzo?",
    "A. Pope Sixtus IV"]

def printer(list):
    n = 0
    l = len(list)
    print
    while n < l:
        print list[n]  # <-- Changed this from a return to a print statement
        newline()
        n += 1
    while n == l:
        n += 1

def qanda(x):
    print
    print "Enter 1 for just questions"
    print "Enter 2 for just answers"
    print "Enter 3 for both"
    qa = raw_input("Enter number: ")
    qa = str(qa)
    if qa == '1':
        printer(x[::2])
    elif qa == '2':
        printer(x[1::2])
    elif qa == '3':
        printer(x)
    else:
        print "Not recognized"

def newline():
    raw_input()
    print



def subjectchoice():
    if subject == "1":
        print
        history()
    elif subject == "2":
        print
        science()
    else:
        print 'Not recognized.'
def science():
    print topics_science
    print "Enter 1 for Light"
    print "Enter 2 for X-mas Exam Review"
    topicchoice = raw_input("Enter number: ")
    topicchoice = str(topicchoice)
    if topicchoice == "1":
        qanda(science_light)
    elif topicchoice == "2":
        qanda(science_xmasreview)
    else:
        print "Not recognized"
        sys.exit
def history():
    print topics_history
    print "Enter 1 for Italian Renaissance"
    topicchoice = raw_input("Enter number: ")
    topicchoice = str(topicchoice)
    if topicchoice == "1":
        return qanda(history_renaissance)
    else:
        print "Not recognized"
        sys.exit()
print subjects
print "Enter 1 for History"
print "Enter 2 for Science"
subject = raw_input("Enter number: ")
subject = str(subject)
subjectchoice()