import string
class PartialFormatter(string.Formatter):
def __init__(self, missing='~~', bad_fmt='!!'):
self.missing, self.bad_fmt=missing, bad_fmt
def get_field(self, field_name, args, kwargs):
# Handle a key not found
try:
val=super(PartialFormatter, self).get_field(field_name, args, kwargs)
# Python 3, 'super().get_field(field_name, args, kwargs)' works
except (KeyError, AttributeError):
val=None,field_name
return val
def format_field(self, value, spec):
# handle an invalid format
if value==None: return self.missing
try:
return super(PartialFormatter, self).format_field(value, spec)
except ValueError:
if self.bad_fmt is not None: return self.bad_fmt
else: raise
fmt=PartialFormatter()
data = {'n': 3, 'k': 3.141594, 'p': {'a': '7', 'b': 8}}
print(fmt.format('{n}, {k:.2f}, {p[a]}, {p[b]}', **data))
# 3, 3.14, 7, 8
del data['k']
data['p']['b'] = None
print(fmt.format('{n}, {k:.2f}, {p[a]:.2f}, {p[b]}', **data))
# 3, ~~, !!, ~~
按照设置，~~如果找不到字段或属性，并且!!给定字段值使用了无效格式，它将打印。（如果您要引发默认值错误，请仅使用None关键字参数bad_fmt。）
为了处理丢失的钥匙，你需要子类都get_field赶上KeyError或AttributeError并format_field为失踪键返回默认值。
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