julia - 如何将可能为 `nothing` 的变量插入 Julia 中的字符串中？

Question

我试图通过将变量的值插入到字符串中来在 Julia 中创建一个动态字符串。一切正常，直到今天值返回 nothing 给我留下错误。

如何在字符串中包含 nothing？至少不必经历一些 if n == nothing 的麻烦； n = "None" 我想插入到字符串中的每个变量的东西。

function charge_summary(charges_df)
    if size(charges_df)[1] > 0
        n_charges = size(charges_df)[1]
        total_charges = round(abs(sum(charges_df[:amount])), digits=2)
        avg_charges = round(abs(mean(charges_df[:amount])), digits=2)
        most_frequent_vender = first(sort(by(charges_df, :transaction_description, nrow), :x1, rev=true))[:transaction_description]
        sms_text = """You have $n_charges new transactions, totaling \$$total_charges.
        Your average expenditure is \$$avg_charges.
        Your most frequented vender is $most_frequent_vender.
        """
        return sms_text
    else
        return nothing
    end
end

sms_msg = charge_summary(charges_df)

返回:

ArgumentError: `nothing` should not be printed; use `show`, `repr`, or custom output instead.
string at io.jl:156 [inlined]
charge_summary(::DataFrame) at get-summary.jl:18
top-level scope at none:0
include_string(::Module, ::String, ::String, ::Int64) at eval.jl:30
(::getfield(Atom, Symbol("##105#109")){String,Int64,String})() at eval.jl:91
withpath(::getfield(Atom, Symbol("##105#109")){String,Int64,String}, ::String) at utils.jl:30
withpath at eval.jl:46 [inlined]
#104 at eval.jl:90 [inlined]
hideprompt(::getfield(Atom, Symbol("##104#108")){String,Int64,String}) at repl.jl:76
macro expansion at eval.jl:89 [inlined]
(::getfield(Atom, Symbol("##103#107")))(::Dict{String,Any}) at eval.jl:84
handlemsg(::Dict{String,Any}, ::Dict{String,Any}) at comm.jl:168
(::getfield(Atom, Symbol("##14#17")){Array{Any,1}})() at task.jl:259

Answer 1

不幸的是，您必须显式处理nothing。例如像这样:

Your most frequented vender is $(something(most_frequent_vender, "None")).

这样做的原因是不清楚你希望如何将 nothing 转换为字符串，所以你必须提供这个值(在你的情况下你想要 "None ").

一个较短的版本是:

Your most frequented vender is $(repr(most_frequent_vender)).

但是 nothing 被打印为 "nothing"。