If I write in Python:
data = {'n': 3, 'k': 3.141594, 'p': {'a': 7, 'b': 8}}
print('{n}, {k:.2f}, {p[a]}, {p[b]}'.format(**data))
del data['k']
data['p']['b'] = None
print('{n}, {k:.2f}, {p[a]}, {p[b]}'.format(**data))
I get:
3, 3.14, 7, 8
Traceback (most recent call last):
File "./funky.py", line 186, in <module>
print('{n}, {k:.2f}, {p[a]}, {p[b]}'.format(**data))
KeyError: 'k'
Instead of an error message, how can I get Python to more gracefully format the None's and non existent fields?
To give an example, I would like to see in the output something more like:
3, 3.14, 7, 8
3, ~, 7, ~
Ideally, of course, I would like to be able to specify the string used instead of those missing values.
The recommendation in PEP 3101 is to subclass Formatter:
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, ~~, !!, ~~
As set up, it will print ~~
if a field or attribute is not found and !!
if an invalid format is used given the field value. (Just use None
for the keyword argument bad_fmt
if you want the default of a value error raised.)
To handle missing keys, you need to subclass both get_field
to catch the KeyError
or AttributeError
and format_field
to return a default value for the missing key.
Since you are catching format_field
errors, you can catch a bad format field as well by catching the ValueError
from the superclass.