Handling KeyError: key “nameOfKey” not found

I have a script that looks through a JSON structure. Every now and again, a particular key is missing. When these instances occur, I need the script to continue rather than stop. Here is my code:

for i = 1:length(marketCatalogueResult)
        marketResults = getMarketOffers(currentCatalogue[i,1])
        push!(lastTradeP1, marketResults[1]["runners"][1]["lastPriceTraded"])
        push!(lastTradeP2, marketResults[1]["runners"][2]["lastPriceTraded"])

The error I sometimes get is:

KeyError: key "lastPriceTraded" not found

How can I amend my code to handle the stated error?

What do you want the code to do in the case when the key doesn’t exist?


Insert NaN

You can use the get function to get either the value associated with a key or some default if that key does not exist. For example:

julia> results = Dict("lastPrice" => 1.0, "currentPrice" => 2.0)
Dict{String,Float64} with 2 entries:
  "currentPrice" => 2.0
  "lastPrice"    => 1.0

julia> get(results, "lastPrice", NaN)

julia> get(results, "futurePrice", NaN)

Try running ?get for more information.


You may want to consider using missing instead, which is the Julian way to indicate missing data. NaN has a very specific meaning, which is that it is the result of a computation where zero was divided by zero or infinity was subtracted from itself.


While I agree with the semantics, the temptation of just keeping things simple with a single primitive type, with no unions or risk of type instability should not be underestimated. I have some data wrangling code that deal with Float64 columns that will never have a “true” NaN (things like X_time where X is a procedure that is not always called) so it is very useful to just use NaNs as a sentinel value.

Good point. NaN is also a good choice here.

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When I attempt to implement that solution, I get an error to do with float:

Anything obvious I am doing wrong?

It seems marketResults is an Array{Any, 1}, not a Dict?

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Looks like you need something like this:

get(marketResults[1]["runners"][1], "lastPriceTraded", NaN)

You can’t do the indexing and then call get, you replace the indexing with the call to get.