python variable unpacking fails

Question

7.10 and for some reason the following code snippet is producing an error...

device_info = {'username': 'test', 'password': 'test', 'appliance': 'name', 'hostname': 'hostname', 'prodcut': 'juice'}
print "{} {} {} {} {}".format(**device_info)

This raises an exception:

Traceback (most recent call last):
  File "python", line 4, in <module>
    print "{} {} {} {} {}".format(**device_info)
IndexError: tuple index out of range

I believe this code should be syntactically sound, however, I don't seem to be able to unpack my dict to pass to any functions, not sure why that doesn't work.

What is the expected output? The first `*` will just put the keys in there, so the second `*` is going to cause a syntax error — Easton Bornemeier, Jul 10 '18 at 16:23

Martijn Pieters · Accepted Answer · 2018-07-10T16:28:51.780

You are passing in your fields as keyword arguments, because you are using ** syntax:

"....".format(**device_info)
#             ^^

Your placeholders, however, specifically only work with positional arguments; placeholders without any name or index, are automatically numbered:

"{} {} {} {} {}".format(...)
# ^0 ^1 ^2 ^3 ^4

This is why you get an index error, there is no positional argument with index 0. Keyword arguments have no index, because they are essentially key-value pairs in a dictionary, which are unordered structures.

If you wanted to include the values from the dictionary into a string, then you need to explicitly name your placeholders:

"{username} {password} {appliance} {hostname} {prodcut}".format(**device_info)

(but note the misspelling of product as prodcut, you may want to check if your dictionary keys are spelled correctly).

You'd get all the values inserted in the named slots:

>>> print "{username} {password} {appliance} {hostname} {prodcut}".format(**device_info)
test test name hostname juice

If you expected the keys to be printed, then you'd have to pass in the device_info keys as separate positional arguments; "...".format(*device_info) (single *) would do just that, but then you'd also have to content with the 'arbitrary' dictionary order that the keys would be listed in.

score -1 · Answer 2 · answered Jul 10 '18 at 16:58

-1

device_info = {'username': 'test', 'password': 'test', 'appliance': 'name', 'hostname': 'hostname', 'prodcut': 'juice'}

print ("{username} {password} {appliance} {hostname} {prodcut}".format(**device_info))

answered Jul 10 '18 at 16:58

kanishk

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python variable unpacking fails

2 Answers2