Python String Format Suppress/silent Keyerror/indexerror

Is there a way to use python string.format such that no exception is thrown when an index is missing, instead an empty string is inserted. result = 'i am an {error} example string

Solution 1:

str.format() doesn't expect a mapping object. Try this:

from collections import defaultdict

d = defaultdict(str)
d['error2'] = "success"
s = "i am an {0[error]} example string {0[error2]}"print s.format(d)

You make a defaultdict with a str() factory that returns "". Then you make one key for the defaultdict. In the format string, you access keys of the first object passed. This has the advantage of allowing you to pass other keys and values, as long as your defaultdict is the first argument to format().

Also, see http://bugs.python.org/issue6081

Solution 2:

The official solution (Python 3 Docs) for strings in format mappings is to subclass the dict class and to define the magic-method __missing__(). This method is called whenever a key is missing, and what it returns is used for the string formatting instead:

classformat_dict(dict):
    def__missing__(self, key):
        return"..."

d = format_dict({"foo": "name"})

print("My %(foo)s is %(bar)s" % d) # "My name is ..."print("My {foo} is {bar}".format(**d)) # "My name is ..."

Edit: the second print() works in Python 3.5.3, but it does not in e.g. 3.7.2: KeyError: 'bar' is raised and I couldn't find a way to catch it.

After some experiments, I found a difference in Python's behavior. In v3.5.3, the calls are __getitem__(self, "foo") which succeeds and __getitem__(self, "bar") which can not find the key "bar", therefore it calls __missing__(self, "bar") to handle the missing key without throwing a KeyError. In v3.7.2, __getattribute__(self, "keys") is called internally. The built-in keys() method is used to return an iterator over the keys, which yields "foo", __getitem__("foo") succeeds, then the iterator is exhausted. For {bar} from the format string there is no key "bar". __getitem__() and hence __missing_() are not called to handle the situation. Instead, the KeyError is thrown. I don't know how one could catch it, if at all.

In Python 3.2+ you should use format_map() instead (also see Python Bug Tracker - Issue 6081):

from collections import defaultdict
    
d = defaultdict(lambda: "...")
d.update({"foo": "name"})

print("My {foo} is {bar}".format_map(d)) # "My name is ..."

If you want to keep the placeholders, you can do:

classDefault(dict):
    def__missing__(self, key): 
        return key.join("{}")
    
d = Default({"foo": "name"})

print("My {foo} is {bar}".format_map(d)) # "My name is {bar}"

As you can see, format_map() does call __missing__().

The following appears to be the most compatible solution as it also works in older Python versions including 2.x (I tested v2.7.15):

classDefault(dict):
    def__missing__(self, key):
        return key.join("{}")

d = Default({"foo": "name"})

import string
print(string.Formatter().vformat("My {foo} is {bar}", (), d)) # "My name is {bar}"

To keep placeholders as-is including the format spec (e.g. {bar:<15}) the Formatter needs to be subclassed:

import string

classUnformatted:
    def__init__(self, key):
        self.key = key
    def__format__(self, format_spec):
        return"{{{}{}}}".format(self.key, ":" + format_spec if format_spec else"")

classFormatter(string.Formatter):
    defget_value(self, key, args, kwargs):
        ifisinstance(key, int):
            try:
                return args[key]
            except IndexError:
                return Unformatted(key)
        else:
            try:
                return kwargs[key]
            except KeyError:
                return Unformatted(key)


f = Formatter()
s1 = f.vformat("My {0} {1} {foo:<10} is {bar:<15}!", ["real"], {"foo": "name"})
s2 = f.vformat(s1, [None, "actual"], {"bar":"Geraldine"})
print(s1) # "My real {1} name       is {bar:<15}!"print(s2) # "My real actual name       is Geraldine      !"

Note that the placeholder indices are not changed ({1} remains in the string without a {0}), and in order to substitute {1} you need to pass an array with any odd first element and what you want to substitute the remaining placeholder with as second element (e.g. [None, "actual"]).

You can also call the format() method with positional and named arguments:

s1 = f.format("My {0} {1} {foo:<10} is {bar:<15}!", "real", foo="name")
s2 = f.format(s1, None, "actual", bar="Geraldine")

Solution 3:

Unfortunately, no, there is no such way to do by default. However you can provide it defaultdict or object with overridden __getattr__, and use like this:

classSafeFormat(object):
    def__init__(self, **kw):
        self.__dict = kw

    def__getattr__(self, name):
        ifnot name.startswith('__'):
            return self.__dict.get(name, '')

print"i am an {0.error} example string {0.error2}".format(SafeFormat(hello=2,error2="success"))
i am an  example string success

Solution 4:

I made a version that does work similarly to Daniel's method but without the {0.x} attribute access.

import string    
classSafeFormat(object):
    def__init__(self, **kw):
        self.__dict = kw

    def__getitem__(self, name):
        return self.__dict.get(name, '{%s}' % name)



string.Formatter().vformat('{what} {man}', [], SafeFormat(man=2))

prints out

'{what} 2'