youknowone · Oct 10, 2026 · Oct 10, 2026
diff --git a/crates/capi/src/bytesobject.rs b/crates/capi/src/bytesobject.rs
@@ -1,7 +1,7 @@
 use crate::object::define_py_check;
 use crate::util::FfiPtrExt;
 use crate::{PyObject, pystate::with_vm};
-use core::ffi::{CStr, c_char, c_int};
+use core::ffi::{CStr, VaList, c_char, c_int, c_long, c_uint, c_ulong, c_void};
 use rustpython_vm::builtins::PyBytes;
 
 define_py_check!(fn PyBytes_Check, types.bytes_type);
@@ -38,6 +38,136 @@ pub unsafe extern "C" fn PyBytes_FromString(s: *const c_char) -> *mut PyObject {
     })
 }
 
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn PyBytes_FromFormat(format: *const c_char, args: ...) -> *mut PyObject {
+    unsafe { PyBytes_FromFormatV(format, args) }
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn PyBytes_FromFormatV(
+    format: *const c_char,
+    mut args: VaList<'_>,
+) -> *mut PyObject {
+    with_vm(|vm| {
+        let format = unsafe { CStr::from_ptr(format) }.to_bytes();
+        let mut result = Vec::with_capacity(format.len());
+        let mut pos = 0;
+        while pos < format.len() {
+            if format[pos] != b'%' {
+                result.push(format[pos]);
+                pos += 1;
+                continue;
+            }
+            let start = pos;
+            pos += 1;
+            // The C API ignores field width and applies precision only to %s.
+            while format.get(pos).is_some_and(u8::is_ascii_digit) {
+                pos += 1;
+            }
+            let mut precision = 0usize;
+            if format.get(pos) == Some(&b'.') {
+                pos += 1;
+                while let Some(&digit) = format.get(pos).filter(|b| b.is_ascii_digit()) {
+                    precision = precision
+                        .saturating_mul(10)
+                        .saturating_add((digit - b'0').into());
+                    pos += 1;
+                }
+            }
+            while format
+                .get(pos)
+                .is_some_and(|b| *b != b'%' && !b.is_ascii_alphabetic())
+            {
+                pos += 1;
+            }
+            let modifier = if matches!(format.get(pos), Some(b'l' | b'z'))
+                && matches!(format.get(pos + 1), Some(b'd' | b'u'))
+            {
+                let modifier = format[pos];
+                pos += 1;
+                Some(modifier)
+            } else {
+                None
+            };
+            match format.get(pos) {
+                Some(b'c') => {
+                    let value = unsafe { args.next_arg::<c_int>() };
+                    let value = u8::try_from(value).map_err(|_| {
+                        vm.new_overflow_error(
+                            "PyBytes_FromFormatV(): %c format expects an integer in range [0; 255]",
+                        )
+                    })?;
+                    result.push(value);
+                }
+                Some(b'd') => {
+                    let value = match modifier {
+                        Some(b'l') => unsafe { args.next_arg::<c_long>() }.to_string(),
+                        Some(b'z') => unsafe { args.next_arg::<isize>() }.to_string(),
+                        _ => unsafe { args.next_arg::<c_int>() }.to_string(),
+                    };
+                    result.extend_from_slice(value.as_bytes());
+                }
+                Some(b'u') => {
+                    let value = match modifier {
+                        Some(b'l') => unsafe { args.next_arg::<c_ulong>() }.to_string(),
+                        Some(b'z') => unsafe { args.next_arg::<usize>() }.to_string(),
+                        _ => unsafe { args.next_arg::<c_uint>() }.to_string(),
+                    };
+                    result.extend_from_slice(value.as_bytes());
+                }
+                Some(b'i') => result
+                    .extend_from_slice(unsafe { args.next_arg::<c_int>() }.to_string().as_bytes()),
+                Some(b'x') => result.extend_from_slice(
+                    format!("{:x}", unsafe { args.next_arg::<c_int>() } as c_uint).as_bytes(),
+                ),
+                Some(b's') => {
+                    let string = unsafe { args.next_arg::<*const c_char>() };
+                    if precision == 0 {
+                        result.extend_from_slice(unsafe { CStr::from_ptr(string) }.to_bytes());
+                    } else {
+                        // A precision-limited string need not have a terminator
+                        // beyond the specified readable region.
+                        for offset in 0..precision {
+                            let byte = unsafe { *string.add(offset) } as u8;
+                            if byte == 0 {
+                                break;
+                            }
+                            result.push(byte);
+                        }
+                    }
+                }
+                Some(b'p') => {
+                    let pointer = unsafe { args.next_arg::<*const c_void>() };
+                    // Preserve the platform's %p spelling, including Windows
+                    // uppercase digits, but always supply the C API's 0x prefix.
+                    let mut buffer = [0 as c_char; 2 * size_of::<usize>() + 16];
+                    unsafe {
+                        libc::snprintf(buffer.as_mut_ptr(), buffer.len(), c"%p".as_ptr(), pointer);
+                    }
+                    let pointer = unsafe { CStr::from_ptr(buffer.as_ptr()) }.to_bytes();
+                    result.extend_from_slice(b"0x");
+                    result.extend_from_slice(
+                        if pointer.starts_with(b"0x") || pointer.starts_with(b"0X") {
+                            &pointer[2..]
+                        } else {
+                            pointer
+                        },
+                    );
+                }
+                Some(b'%') => result.push(b'%'),
+                _ => {
+                    // The first unrecognized format copies the remaining
+                    // format verbatim, without consuming any more arguments.
+                    result.extend_from_slice(&format[start..]);
+                    break;
+                }
+            }
+            pos += 1;
+        }
+        Ok(vm.ctx.new_bytes(result))
+    })
+}
+
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn PyBytes_FromObject(obj: *mut PyObject) -> *mut PyObject {
     with_vm(|vm| {