如何检测字典中是否存在值？

Question

Here I have code that gives students a math quiz and saves their scores in a text file. Now I'm trying to add to my code to enable it to save the student's last three scores to their name. To do this I know I will have to use 2D arrays, but I'm struggling to add an if statement to my code that essentially says "If name is in dictionary, save score to name". How would I do this?

Here is my code:

import random
import sys

def get_input_or_quit(prompt, quit="Q"):
    prompt += " (Press '{}' to exit) : ".format(quit)
    val = input(prompt).strip()
    if val.upper() == quit:
        sys.exit("Goodbye")
    return val

def prompt_bool(prompt):
    while True:
        val = get_input_or_quit(prompt).lower()
        if val == 'yes':
          return True
        elif val == 'no':
          return False
        else:
         print ("Invalid input '{}', please try again".format(val))


def prompt_int_small(prompt='', choices=(1,2)):
    while True:
        val = get_input_or_quit(prompt)
        try:
            val = int(val)
            if choices and val not in choices:
                raise ValueError("{} is not in {}".format(val, choices))
            return val
        except (TypeError, ValueError) as e:
                print(
                    "Not a valid number ({}), please try again".format(e)
                    )

def prompt_int_big(prompt='', choices=(1,2,3)):
    while True:
        val = get_input_or_quit(prompt)
        try:
            val = int(val)
            if choices and val not in choices:
                raise ValueError("{} is not in {}".format(val, choices))
            return val
        except (TypeError, ValueError) as e:
                print(
                    "Not a valid number ({}), please try again".format(e)
                    )

role = prompt_int_small("Are you a teacher or student? Press 1 if you are a student or 2 if you are a teacher")
if role == 1:
    score=0
    name=input("What is your name?")
    print ("Alright",name,"welcome to your maths quiz."
            " Remember to round all answers to 5 decimal places.")
    level_of_difficulty = prompt_int_big("What level of difficulty are you working at?\n"
                                 "Press 1 for low, 2 for intermediate "
                                    "or 3 for high\n")


    if level_of_difficulty == 3:
        ops = ['+', '-', '*', '/']
    else:
        ops = ['+', '-', '*']

    for question_num in range(1, 11):
        if level_of_difficulty == 1:
            max_number = 10
        else:
            max_number = 20

        number_1 = random.randrange(1, max_number)
        number_2 = random.randrange(1, max_number)

        operation = random.choice(ops)
        maths = round(eval(str(number_1) + operation + str(number_2)),5)
        print('\nQuestion number: {}'.format(question_num))
        print ("The question is",number_1,operation,number_2)
        answer = float(input("What is your answer: "))
        if answer == maths:
            print("Correct")
            score = score + 1
        else:
            print ("Incorrect. The actual answer is",maths)

    if score >5:
        print("Well done you scored",score,"out of 10")
    else:
        print("Unfortunately you only scored",score,"out of 10. Better luck next time")

    class_number = prompt_int_big("Before your score is saved ,are you in class 1, 2 or 3? Press the matching number")

    filename = (str(class_number) + "txt")
    with open(filename, 'a') as f:
        f.write("\n" + str(name) + " scored " + str(score) +  " on difficulty level " + str(level_of_difficulty) + "\n")
    with open(filename) as f:
        lines = [line for line in f if line.strip()]
        lines.sort()

    if prompt_bool("Do you wish to view previous results for your class"):
        for line in lines:
            print (line)
    else:
        sys.exit("Thanks for taking part in the quiz, your teacher should discuss your score with you later")
if role == 2:
    class_number = prompt_int_big("Which class' scores would you like to see? Press 1 for class 1, 2 for class 2 or 3 for class 3")
    filename = (str(class_number) + "txt")

    f = open(filename, "r")
    lines = [line for line in f if line.strip()]
    lines.sort()
    for line in lines:
        print (line)

Here is the section where I am trying to add this if statement:

class_number = prompt_int_big("Before your score is saved ,are you in class 1, 2 or 3? Press the matching number")

    filename = (str(class_number) + "txt")
    with open(filename, 'a') as f:
        f.write("\n" + str(name) + " scored " + str(score) +  " on difficulty level " + str(level_of_difficulty) + "\n")
    with open(filename) as f:
        lines = [line for line in f if line.strip()]
        lines.sort()

Answer 1

If can simply do 'in'

if 'Name' in Variable:
   do stuff

The variable can be a list or dictionary.

Answer 2

Since you anyway read the file content back into lines , if your ultimate goal is to show the last three results, I would rather approach it by extracting the information from there inside your prompt_bool("Do you wish to view previous results for your class") section along the lines of:

if prompt_bool("Do you wish to view previous results for your class"):
  scores_shown=0
  for line in lines[::-1]:
    if name==line.split(' scored ')[0].strip():
      print(line)
      scores_shown+=1
      if scores_shown>=3:
        break

This would not collocate the scores in the same way as a dictionary would however.

You could populate a dictionary of lists of scores doing something like this:

if name in d:
  d[name].append(score)
else:
  d[name]=[score]

you could even have a nested list to include the difficulty level eg: d[name]=[[score,level_of_difficulty]] to make the first list item, then d[name].append([score,level_of_difficulty]) for the subsequent items in a dictionary d .

The code would fail if someone has a name which includes the string ' scored ' .

On a side note, prompt_int_big and prompt_int_small are identical except for the choices tuple. To avoid duplicate code I would set one of them as a default prompt_int and when using the other, just add it as an argument, utilizing the flexibility of your code; eg. prompt_int(choices=(1,2,3)) or prompt_int("my prompt",(1,2,3,4)) . Also "txt" could be replaced with ".txt" to make it easier for some OS's to understand the file type.