added method for FormatLogger to write directly to file and some doc

Project.toml

@@ -1,7 +1,7 @@
 name = "LoggingExtras"
 uuid = "e6f89c97-d47a-5376-807f-9c37f3926c36"
 authors = ["Lyndon White <[email protected]>"]
-version = "0.4.7"
+version = "0.4.8"
 
 [deps]
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"

README.md

@@ -281,6 +281,9 @@ It is really simple.
 The resulting file format is similar to that which is shown in the REPL.
 (Not identical, but similar)
 
+**NOTE**: To print to the file in a specific format, e.g. to create a JSON log, use
+`FormatLogger` instead.
+
 ### Demo: `TeeLogger` and `FileLogger`
 We are going to log info and above to one file,
 and warnings and above to another.
@@ -349,13 +352,20 @@ in the constructor. See `FormatLogger` for the requirements on the formatter fun
 The `FormatLogger` is a sink that formats the message and prints to a wrapped IO.
 Formatting is done by providing a function `f(io::IO, log_args::NamedTuple)`.
 
+The format logger can take as its second argument either the `IO` to be written to or the path
+of a file to be written to.
+
 ```julia
 julia> using LoggingExtras
 
 julia> logger = FormatLogger() do io, args
            println(io, args._module, " | ", "[", args.level, "] ", args.message)
        end;
 
+julia> logger = FormatLogger("out.log") do io, args
+            println(io, args._module, " | ", "[", args.level, "] ", args.message)
+        end;
+
 julia> with_logger(logger) do
            @info "This is an informational message."
            @warn "This is a warning, should take a look."

src/filelogger.jl

@@ -9,6 +9,8 @@ end
 Create a logger sink that write messages to a file specified with `path`.
 To append to the file (rather than truncating the file first), use `append=true`.
 If `always_flush=true` the stream is flushed after every handled log message.
+
+Note that one should use `FormatLogger` instead to print to the file in a specific format.
 """
 function FileLogger(path; append=false, kwargs...)
     filehandle = open(path, append ? "a" : "w")
@@ -22,6 +24,8 @@ Create a logger sink that write messages to the `io::IOStream`. The stream
 is expected to be open and writeable.
 If `always_flush=true` the stream is flushed after every handled log message.
 
+Note that one should use `FormatLogger` instead to print to the file in a specific format.
+
 # Examples
 ```julia
 io = open("path/to/file.log", "a") # append to the file

src/formatlogger.jl

@@ -34,6 +34,20 @@ function FormatLogger(f::Function, io::IO=stderr; always_flush=true)
     return FormatLogger(f, io, always_flush)
 end
 
+"""
+    FormatLogger(f::Function, path::AbstractString; append=false, always_flush=true)
+
+Logger sink that formats the message and writes it to the file at `path`.  This is similar
+to `FileLogger` except that it allows specifying the printing format.
+
+To append to the file (rather than truncating the file first), use `append=true`.
+If `always_flush=true` the stream is flushed after every handled log message.
+"""
+function FormatLogger(f::Function, path::AbstractString; append::Bool=false, kw...)
+    io = open(path, append ? "a" : "w")
+    return FormatLogger(f, io; kw...)
+end
+
 function handle_message(logger::FormatLogger, args...; kwargs...)
     log_args = handle_message_args(args...; kwargs...)
     # We help the user by passing an IOBuffer to the formatting function

test/runtests.jl

@@ -221,6 +221,22 @@ end
     logger = FormatLogger(x -> x; always_flush=false)
     @test logger.stream === stderr
     @test !logger.always_flush
+
+    # test file arguments
+    mktempdir() do dir
+        f = joinpath(dir, "test.log")
+
+        logger = FormatLogger(f) do io, args
+            println(io, "log message")
+        end
+
+        with_logger(logger) do
+            @info "test message"
+        end
+
+        l = read(f, String)
+        @test startswith(l, "log message")
+    end
 end
 
 @testset "Deprecations" begin