@printf should use textwidth for "%ns" and "%0.ns"

[stevengj]

This seems wrong:

julia> @printf("%5s", "foo")
  foo

julia> @printf("%5s", "föó")
föó

julia> @printf("%.2s", "föó")
fo

It seems like @printf (in this code) should really use textwidth to compute string widths for formatting purposes, similar to this discussion on discourse.

(To improve backward compatibility, I suppose we could use textwidth only for %S, which is currently a rarely used synonym for %s, or use another format code entirely.)

[KristofferC]

Related: #38256

[stevengj]

Python 3 also gets this wrong:

>>> print('%0.2s' % 'föó')
fo
>>> f'{"föó": <{5}}'
'föó'
>>> f'{"foo": <{5}}'
'foo  '

[ArunS-tack]

I think even with textwidth, it's giving out the wrong output.

julia> @printf("%5s", "föo")
  fö
julia> @printf("%5s", "foo")
  foo
julia> @printf("%.2s", "föó")
fo

[stevengj]

@arunsanganal, it works if you use textwidth correctly. I get the correct output:

julia> @printf("%5s", "föo")
  föo
julia> @printf("%5s", "foo")
  foo
julia> Printf.@printf("%.2s", "föó")
fö

with

diff --git a/stdlib/Printf/src/Printf.jl b/stdlib/Printf/src/Printf.jl
index 5f42df50b8..282d0ebdfd 100644
--- a/stdlib/Printf/src/Printf.jl
+++ b/stdlib/Printf/src/Printf.jl
@@ -239,7 +239,7 @@ end
 @inline function fmt(buf, pos, arg, spec::Spec{T}) where {T <: Strings}
     leftalign, hash, width, prec = spec.leftalign, spec.hash, spec.width, spec.precision
     str = string(arg)
-    slen = length(str) + (hash ? arg isa AbstractString ? 2 : 1 : 0)
+    slen = textwidth(str) + (hash ? arg isa AbstractString ? 2 : 1 : 0)
     op = p = prec == -1 ? slen : min(slen, prec)
     if !leftalign && width > p
         for _ = 1:(width - p)
@@ -259,9 +259,9 @@ end
         end
     end
     for c in str
-        p == 0 && break
+        p -= textwidth(c)
+        p < 0 && break
         pos = writechar(buf, pos, c)
-        p -= 1
     end
     if hash && arg isa AbstractString && p > 0
         buf[pos] = UInt8('"')
@@ -753,8 +753,9 @@ plength(f::Spec{Pointer}, x) = max(f.width, 2 * sizeof(x) + 2)
 
 function plength(f::Spec{T}, x) where {T <: Strings}
     str = string(x)
-    p = f.precision == -1 ? (length(str) + (f.hash ? (x isa Symbol ? 1 : 2) : 0)) : f.precision
-    return max(f.width, p) + (sizeof(str) - length(str))
+    sw = textwidth(str)
+    p = f.precision == -1 ? (sw + (f.hash ? (x isa Symbol ? 1 : 2) : 0)) : f.precision
+    return max(f.width, p) + (sizeof(str) - sw)
 end
 
 function plength(f::Spec{T}, x) where {T <: Ints}

[ArunS-tack]

Ah! i had missed the edit on line 259. It does work fine now. 😄

[stevengj]

I'll have a PR shortly.