如何在“打印语句”之前在for循环中执行“if语句”

Question

我对 Python 中的文件处理和 csv 文件相当陌生。

代码

import csv
file1=open("Employee.csv","w",newline='\r\n')
empwriter=csv.writer(file1)
empwriter.writerow(["EmpID.","Name","MobNo."])
n=int(input("How many employee records do you want to add?:"))
for i in range(n):
    print("\nEmployee Record",(i+1))
    empid=input("Enter empID:")
    name=input("Enter Name:")
    mob=input("Enter Mobile No.:")
    emprec=[empid,name,mob]
    empwriter.writerow(emprec)
file1.close()

file2=open("Employee.csv","r+",newline='\r\n')
empreader=csv.reader(file2)
newwriter=csv.writer(file2)
check=input("\nEnter the empID to check:\n")
opt=int(input('''Which of the following fields do you want to update?:
1.Update the Name
2.Update the Mobile No.
3Update the whole record\n\n'''))

if opt==1:
    for x in empreader:
        if x[0]==check:
            newname=input("Enter new name:")
            x[1]=newname
            print("Record Updated Successfully!\n Updated record:\n",x)
            newwriter.writerow(x)
        print(x)
elif opt==2:
    for x in empreader:
        if x[0]==check:
            newmob=input("Enter new Mobile No.:")
            x[2]=newmob
            print("Record Updated Successfully!\n Updated record:\n",x)
            newwriter.writerow(x)
        print(x)     
elif opt==3:
    for x in empreader:
        if x[0]==check:
            newname=input("Enter new name:")
            newmob=input("Enter new Mobile No.:")
            x[1]=newname
            x[2]=newmob
            print("Record Updated Successfully!\n Updated record:\n",x)
            newwriter.writerow(x)
        print(x)
file2.close()

我曾在这段代码中尝试过

1. 按用户输入记录（empID、Name、MobNo.）并制作一个 csv 文件。
2. 使用 empID 查找所需记录。
3. 更新该记录。
4. 显示所有记录。

当我执行代码时，for 循环中的 print 语句在 if 语句之前执行并给我这个输出。

输出

How many employee records do you want to add?:3

Employee Record 1
Enter empID:001
Enter Name:John
Enter Mobile No.:1234567890

Employee Record 2
Enter empID:002
Enter Name:Jane
Enter Mobile No.:2345678901

Employee Record 3
Enter empID:003
Enter Name:Judy
Enter Mobile No.:4567890123

Enter the empID to check:
002
Which of the following fields do you want to update?:
1.Update the Name
2.Update the Mobile No.
3.Update the whole record

2
['EmpID.', 'Name', 'MobNo.']
['001', 'John', '1234567890']
Enter new Mobile No.:1111222233
Record Updated Successfully!
 Updated record:
 ['002', 'Jane', '1111222233']
['002', 'Jane', '1111222233']

正如您在输出的最后几行中看到的那样。打印语句在 if 语句（或类似语句）之前执行。我实际上希望输出以下面给定的方式显示...

期望的输出

2
Enter new Mobile No.:1111222233
Record Updated Successfully!
 Updated record:
 ['002', 'Jane', '1111222233']
['EmpID.', 'Name', 'MobNo.']
['001', 'John', '1234567890']
['002', 'Jane', '1111222233']
['003', 'Judy', '4567890123']

Answer 1

我可以看到正在发生的事情，以及为什么它不是您所期望的。

看看这个块：

elif opt==2:
    for x in empreader:
        if x[0]==check:
            newmob=input("Enter new Mobile No.:")
            x[2]=newmob
            print("Record Updated Successfully!\n Updated record:\n",x)
            newwriter.writerow(x)
        print(x)  

实际情况是，对于x 中的每一行empreader：

1. ID col x[0] 正在根据您输入的 ID check 进行评估。 如果匹配：
   1. 你中断循环，并介入修改x
   2. 写出新值newwriter.writerow(x)
   3. 然后你走出去......
2. 打印您的更新，print(x)

除非x[0]==check 是False，在这种情况下x 只是被打印出来。

考虑（并测试）当您为 check 提供不存在的 ID 时会发生什么...它永远不会进入您的“修改界面”，而只会打印 @987654334 中的每个（未修改的）行@。

然后会发生什么，您是否显示错误消息让用户知道他们选择了错误的 ID？

这是一个相当大的变化，但我认为它使逻辑和您的意图更加清晰......您不是在循环中做出决定，这非常困难。以“平坦”的顺序接受输入并做出决定：

import csv
import sys

# Get the employee ID
check=input("\nEnter the empID to check:\n")

# and check the ID in the employee records
emp_row = None
with open("Employee.csv","r",newline='\r\n') as f:
    empreader=csv.reader(f)

    for x in empreader:
        if x[0] == check:  # the employee ID matches
            emp_row = x[:]

if emp_row is None:
    print(f'Could not find employee ID {check}')
    sys.exit(0)  # or exit(1) to show it's an "error"

# Now you know you have a valid ID, because emp_row has data

opt=int(input('''Which of the following fields do you want to update?:
1.Update the Name
2.Update the Mobile No.
3.Update the whole record\n\n'''))

if opt == 1:
    newname = input("Enter new name:")
    emp_row[1] = newname
# else if opt == 2:
#    ...
# else if opt ==3:
#    ...

# Now how do you overwrite the old row in Employee.csv with emp_row?
# Don't try to "modify" the original file as you're reading it, instead...
# Read from the old file, copying all the old rows, except for emp_row, to a new list

new_rows = []
with open('Employee.csv', 'r', newline='\r\n') as f_in:
    reader = csv.reader(f_in)
    header_row = next(reader)
    new_rows.append(header_row)

    for row in reader:
        if row[0] == check:
            new_rows.append(emp_row)
        else:
            new_rows.append(row)

# Then write your new_rows out
with open('Employee.csv', 'w', newline='\r\n') as f_out:
    writer = csv.writer(f_out)
    writer.writerows(new_rows)

我从 Employee.csv 开始，看起来像这样：

EmpID.,Name,MobNo.
001,John,1234567890
002,Jane,2345678901
003,Judy,4567890123

我运行了它，它看起来像这样：

% python3 main.py

Enter the empID to check:
002
Which of the following fields do you want to update?:
1.Update the Name
2.Update the Mobile No.
3.Update the whole record

1
Enter new name:Alice

和 Employee.csv 现在看起来像：

EmpID.,Name,MobNo.

001,John,1234567890

002,Alice,2345678901

003,Judy,4567890123

（我在 Mac 上，所以 \r\n 添加行）