How to reference the empty string key in the Format String Syntax?

Question

The empty string ('') is a pertectly valid key for a dictionary, but I can not reference it using the Format String Syntax

data = { 'a' : 'hello' , '' : 'bye' }
print '{a:<14s}'.format(**data)
print '{:<14s}'.format(**data)

Which outputs:

hello         
Traceback (most recent call last):
  File "xxx.py", line 3, in <module>
    print '{:<14s}'.format(**data)
IndexError: tuple index out of range

Is there any way of referencing that key ... as a dictionary key! I can not convert the data to tuples; a bit of background: I am doing some auto-formatting based on a generic format spec which gets converted to Format String Syntax using dicts as data for the formatting. That is, I can not do this:

print '{0:<14s}'.format(data[''])

The data must always be passed to format as **data (basically, because I am doing a generic .format(**data) in my formatter class)

Martijn Pieters · Accepted Answer · 2015-05-11 16:22:21Z

7

You can't use an empty string. The format strictly limits keys to valid Python identifiers, which means they have to have at least 1 letter or underscore at the start.

From the grammar in the Format String Syntax documentation:

replacement_field ::=  "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name        ::=  arg_name ("." attribute_name | "[" element_index "]")*
arg_name          ::=  [identifier | integer]

So the field_name is either an integer or a valid Python identifier.

Note that empty strings are not the only stings that are not valid identifiers; you cannot use strings with spaces in it either, or strings that start with a digit. Such strings can be used in a dictionary, just not as keyword arguments in Python code nor as field names in string formats.

answered May 11, 2015 at 16:22

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

blueFast Over a year ago

Is there any standard way of "sanitizing" dictionary keys so that they are valid python identifiers? A kind of "default transformation" that converts spaces to underscores, and other trickery so that from an arbitrary dictionary key we get a (similar) valid python identifier? Something like: '' -> '_', '0' -> '_0', 'key with spaces' -> 'key_with_spaces'. I understand this is very specific and probably impossible to get foolproof (collissions are to be expected), but I am probably not the first facing this problem, so maybe a half-backed solution is already out there?

Martijn Pieters Over a year ago

@jeckyll2hide: Not that I am aware of, but it's not that hard: Prefix with a _ if the first character is a digit, replace everything that's not a letter, digit or underscore with something that is a letter, digit or underscore. Optional: append digits to make the value unique.

QuantumMecha · Accepted Answer · 2021-11-12 02:46:34Z

1

An 'arg_name' can be an identifier, so you can assign the empty string to a variable and use it as a key in a dictionary. This example assumes that the RETURN key is pressed to produce the empty string for a comparison:

empty = ''

test = {
    "notso" : "You pressed some other key",
    empty: "You pressed the RETURN key"
    }

char = input("Press the RETURN key: ")
print(test[empty]) if char == empty else print(test["notso"])

edited Nov 12, 2021 at 2:46

QuantumMecha

1,5417 silver badges22 bronze badges

answered Nov 12, 2021 at 0:58

Bill Stratton

211 bronze badge

Collectives™ on Stack Overflow

How to reference the empty string key in the Format String Syntax?

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related