add pyiterate (#594)

* add pyiterate

* add PyIterator

* Skip PyIterator tests on old julia versions

* fix IteratorSize

* fix

* fix

* compute IteratorSize in PyIterator constructor

* fix

* fix

* Update src/pyiterator.jl

Co-Authored-By: jw3126 <jw3126@gmail.com>

* Update src/pyiterator.jl

Co-Authored-By: jw3126 <jw3126@gmail.com>

* Update src/pyiterator.jl

Co-Authored-By: jw3126 <jw3126@gmail.com>

* Update src/pyiterator.jl

Co-Authored-By: jw3126 <jw3126@gmail.com>

* fix

* fix

* Update pyiterator.jl

Author: jw3126

src/pyiterator.jl

@@ -3,6 +3,7 @@
 #########################################################################
 # Iterating over Python objects in Julia
 
+Base.IteratorSize(::Type{PyObject}) = Base.SizeUnknown()
 function _start(po::PyObject)
     sigatomic_begin()
     try
@@ -29,16 +30,81 @@ end
 
     Base.done(po::PyObject, s) = ispynull(s[1])
 else
-    function Base.iterate(po::PyObject, s=_start(po))
+    """
+        PyIterator{T}(pyobject)
+
+    Wrap `pyobject::PyObject` into an iterator, that produces items of type `T`. To be more precise `convert(T, item)` is applied in each iteration. This can be useful to avoid automatic conversion of items into corresponding julia types.
+    ```jldoctest
+    julia> using PyCall
+
+    julia> l = PyObject([PyObject(1), PyObject(2)])
+    PyObject [1, 2]
+
+    julia> piter = PyCall.PyIterator{PyAny}(l)
+    PyCall.PyIterator{PyAny,Base.HasLength()}(PyObject [1, 2])
+
+    julia> collect(piter)
+    2-element Array{Any,1}:
+     1
+     2
+
+    julia> piter = PyCall.PyIterator(l)
+    PyCall.PyIterator{PyObject,Base.HasLength()}(PyObject [1, 2])
+
+    julia> collect(piter)
+    2-element Array{PyObject,1}:
+     PyObject 1
+     PyObject 2
+    ```
+    """
+    struct PyIterator{T,S}
+        o::PyObject
+    end
+
+    function _compute_IteratorSize(o::PyObject)
+        S = try
+            length(o)
+            Base.HasLength
+        catch err
+            if !(err isa PyError && pyisinstance(err.val, @pyglobalobjptr :PyExc_TypeError))
+                rethrow()
+            end
+            Base.SizeUnknown
+        end
+    end
+    function PyIterator(o::PyObject) 
+        PyIterator{PyObject}(o)
+    end
+    function (::Type{PyIterator{T}})(o::PyObject) where {T}
+        S = _compute_IteratorSize(o)
+        PyIterator{T,S}(o)
+    end
+    
+    Base.eltype(::Type{<:PyIterator{T}}) where T = T
+    Base.eltype(::Type{<:PyIterator{PyAny}}) = Any
+    Base.length(piter::PyIterator) = length(piter.o)
+
+    Base.IteratorSize(::Type{<: PyIterator{T,S}}) where {T,S} = S()
+
+    _start(piter::PyIterator) = _start(piter.o)
+    
+    function Base.iterate(piter::PyIterator{T}, s=_start(piter)) where {T}
         ispynull(s[1]) && return nothing
         sigatomic_begin()
         try
             nxt = PyObject(@pycheck ccall((@pysym :PyIter_Next), PyPtr, (PyPtr,), s[2]))
-            return (convert(PyAny, s[1]), (nxt, s[2]))
+            return (convert(T,s[1]), (nxt, s[2]))
         finally
             sigatomic_end()
         end
     end
+    function Base.iterate(po::PyObject, s=_start(po))
+        # avoid the constructor that calls length
+        # since that might be an expensive operation
+        # even if length is cheap, this adds 10% performance
+        piter = PyIterator{PyAny, Base.SizeUnknown}(po)
+        iterate(piter, s)
+    end
 end
 
 # issue #216

test/runtests.jl

@@ -687,6 +687,45 @@ def try_call(f):
                        pybuiltin("Exception"))
 end
 
+@static if VERSION < v"0.7.0-DEV.5126" # julia#25261
+    # PyIterator not defined in this julia version
+else
+    @testset "PyIterator" begin
+        arr = [1,2]
+        o = PyObject(arr)
+        c_pyany = collect(PyCall.PyIterator{PyAny}(o))
+        @test c_pyany == arr
+        @test c_pyany[1] isa Integer
+        @test c_pyany[2] isa Integer
+
+        c_f64 = collect(PyCall.PyIterator{Float64}(o))
+        @test c_f64 == arr
+        @test eltype(c_f64) == Float64
+
+        i1 = PyObject([1])
+        i2 = PyObject([2])
+        l = PyObject([i1,i2])
+
+        piter = PyCall.PyIterator(l)
+        @test length(piter) == 2
+        @test length(collect(piter)) == 2
+        r1, r2 = collect(piter)
+        @test r1.o === i1.o
+        @test r2.o  === i2.o
+
+        @test Base.IteratorSize(PyCall.PyIterator(PyObject(1))) == Base.SizeUnknown()
+        @test Base.IteratorSize(PyCall.PyIterator(PyObject([1]))) == Base.HasLength()
+
+        # 594
+        @test collect(zip(py"iter([1, 2, 3])", 1:3)) ==
+        [(1, 1), (2, 2), (3, 3)]
+        @test collect(zip(PyCall.PyIterator{Int}(py"iter([1, 2, 3])"), 1:3)) ==
+        [(1, 1), (2, 2), (3, 3)]
+        @test collect(zip(PyCall.PyIterator(py"[1, 2, 3]"o), 1:3)) ==
+        [(1, 1), (2, 2), (3, 3)]
+    end
+end
+  
 @testset "atexit" begin
     if VERSION < v"0.7-"
         setup = ""