自定义 .format() 的用户输入

Question

我得到了一个包含 4 个变量的元组列表，每个变量将打印在一行中，我希望用户指定这些打印的格式。

到目前为止，这是我的代码：

mylist = [
    ("Monday", "13", "November", "2017"),
    ("Tuesday", "14", "November", "2017"),
    ("Wednesday", "15", "November", "2017")]
# Note: The real list have a thousand of items, not just 3.

custom_format = self.GUI_field_where_user_types.get_text()
for item in mylist:
    weekday = item[0]
    daynumber = item[1]
    month = item[2]
    year = item[3]
    print(eval(custom_format))

# Example custom_format: '"{0}, {1} of {2} from year {3}".format(weekday, daynumber, month, year)'

对于测试，我只是尝试了上面的输入，它显然有效，但我不想使用 eval，因为它显然太危险了。所以我的问题是：如何让用户以安全的方式定义格式？

注意： 用户应该只能编辑 .format() 方法之前的字符串 "{0}, {1} of {2} from year {3}"，阻止访问任何 python 命令，但只允许他使用他想要的变量，而不是总是使用其中的 4 个。一些示例可能是：

{0}，{1}，共 {2}
{3}，{1} 月 {2}
{0}，{3} 年 {2} 中的 {1} 个
{1}随便{2} -- {3}

Answer 1

使用一种方法来排列格式：

def fill_format(custom_format, date):
    used_vars = []
    used_indexes = []

    #Look for what is used
    for i in range(len(date)):
        if "{"+str(i)+"}" in custom_format:
            used_vars.append(date[i])
            used_indexes.append(i)

    #Replace indexes starting from 0
    for new_index, used_index in enumerate(used_indexes):
        custom_format = custom_format.replace("{"+str(used_index)+"}", "{"+str(new_index)+"}")

    return custom_format.format(*used_vars)

mylist = [
    ("Monday", "13", "November", "2017"),
    ("Tuesday", "14", "November", "2017"),
    ("Wednesday", "15", "November", "2017")]

custom_format = "{3}, {1} at month {2}"
for item in mylist:
    print(fill_format(custom_format, item))

输出：

2017, 13 at month November
2017, 14 at month November
2017, 15 at month November

编辑：

如果一个日期总是有 4 个时间单位，您可以使用：

def fill_format(custom_format, date):
    return custom_format.replace("{0}", date[0]).replace("{1}", date[1]).replace("{2}", date[2]).replace("{3}", date[3])

Answer 2

我的建议：坚持使用标准化的 datetime 格式。借助 datetime，我们可以使用强大的 strftime 根据一组预定义的模式打印日期时间。考虑这个例子：

import datetime

mylist = [
    ("Monday", "13", "November", "2017"),
    ("Tuesday", "14", "November", "2017"),
    ("Wednesday", "15", "November", "2017")]

custom_format = "%A, %d of %B from year %Y"

for item in mylist:
    # Create datetime object
    dt = datetime.datetime.strptime('-'.join([*item]),"%A-%d-%B-%Y")
    # Print with format
    print(dt.strftime(custom_format))

打印：

Monday, 13 of November from year 2017
Tuesday, 14 of November from year 2017
Wednesday, 15 of November from year 2017

如果您想创建自己的 "own language"，请使用字典映射到标准字典。考虑这个例子：

import datetime
import re

mylist = [
    ("Monday", "13", "November", "2017"),
    ("Tuesday", "14", "November", "2017"),
    ("Wednesday", "15", "November", "2017")]

d = {"WEEKDAY": "%A",
     "DAYNUMBER": "%d",
     "MONTH": "%B",
     "YEAR": "%Y"}

input_ = "YEAR-%m-DAYNUMBER is equal to WEEKDAY, DAYNUMBER of MONTH from year YEAR"

# Use re library to change WEEKDAY --> %A and so on...
pattern = re.compile(r'\b(' + '|'.join(d.keys()) + r')\b')
custom_format = pattern.sub(lambda x: d[x.group()], input_)

for item in mylist:
    # Create datetime object
    dt = datetime.datetime.strptime('-'.join([*item]),"%A-%d-%B-%Y")
    # Print with format
    print(dt.strftime(custom_format))

输出：

2017-11-13 is equal to Monday, 13 of November from year 2017
2017-11-14 is equal to Tuesday, 14 of November from year 2017
2017-11-15 is equal to Wednesday, 15 of November from year 2017

这以最少的代码提供了您需要的所有灵活性。

Answer 3

很简单。只需在输入字符串上调用 .format，并使用参数解包语法将项目作为参数传递：

mylist = [
    ("Monday", "13", "November", "2017"),
    ("Tuesday", "14", "November", "2017"),
    ("Wednesday", "15", "November", "2017")]

fmt = input('Format? ')
for item in mylist:
    print(fmt.format(*item))

执行：

Format? {0}, {1} of {2}
Monday, 13 of November
Tuesday, 14 of November
Wednesday, 15 of November

要处理用户输入无效的格式，您可以捕获异常并重试：

while True:
    try:
        fmt = input('Format? ')
        for item in mylist:
            print(fmt.format(*item))
        break
    except (IndexError,ValueError,KeyError):
        print('Invalid format.')

执行：

Format? {A}
Invalid format.
Format? {4}
Invalid format.
Format? {
Invalid format.
Format? The year is {3}
The year is 2017
The year is 2017
The year is 2017

自定义 .format() 的用户输入

User input for custom .format()

python

eval

python-3.x