str.format() raises KeyError

Question

The following code raises a KeyError exception:

addr_list_formatted = []
addr_list_idx = 0

for addr in addr_list: # addr_list is a list
    addr_list_idx = addr_list_idx + 1
    addr_list_formatted.append("""
        "{0}"
        {
        "gamedir"  "str"
        "address"  "{1}"
        }
    """.format(addr_list_idx, addr))

Why?

I am using Python 3.1.

Solution

The problem is those { and } characters you have there that don't specify a key for formatting. You need to double them up, so change your code to:

addr_list_formatted.append("""
    "{0}"
    {{
    "gamedir"  "str"
    "address"  "{1}"
    }}
""".format(addr_list_idx, addr))

Comments

[fijiaaron]

What if someone wanted to use JSON in Python?

[mgc]

@fijiaaron the double { in the answer is just to tell the format method that there is no key to format here (so they are escaped in the formated string and it shouldn't be a problem for build a JSON that way). Alternatively there is other efficient ways to manipulate strings, like the join method : "".join(['{"', var_name, '":', value, '}'])

[Dmitry Kankalovich]

I almost lost my mind until figured whats the problem

[Lasse V. Karlsen]

@DmitryKankalovich You have lost your mind, Stack Overflow, and everybody here, is just a figment of your imagination. (that's what I keep telling myself at least) :)