diff --git a/py/lexerunix.c b/py/lexerunix.c
index 73d30f2476..52fbe740ae 100644
--- a/py/lexerunix.c
+++ b/py/lexerunix.c
@@ -13,23 +13,45 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 
-mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
-    int fd = open(filename, O_RDONLY);
-    if (fd < 0) {
-        return NULL;
-    }
-    uint size = lseek(fd, 0, SEEK_END);
-    lseek(fd, 0, SEEK_SET);
-    char *data = m_new(char, size);
-    int read_size = read(fd, data, size);
-    close(fd);
-    if (read_size != size) {
-        printf("error reading file %s\n", filename);
-        m_del(char, data, size);
-        return NULL;
-    }
+typedef struct _mp_lexer_file_buf_t {
+    int fd;
+    char buf[20];
+    uint len;
+    uint pos;
+} mp_lexer_file_buf_t;
 
-    return mp_lexer_new_from_str_len(qstr_from_str(filename), data, size, size);
+STATIC unichar file_buf_next_char(mp_lexer_file_buf_t *fb) {
+    if (fb->pos >= fb->len) {
+        if (fb->len < sizeof(fb->buf)) {
+            return MP_LEXER_CHAR_EOF;
+        } else {
+            int n = read(fb->fd, fb->buf, sizeof(fb->buf));
+            if (n <= 0) {
+                return MP_LEXER_CHAR_EOF;
+            }
+            fb->len = n;
+            fb->pos = 0;
+        }
+    }
+    return fb->buf[fb->pos++];
+}
+
+STATIC void file_buf_close(mp_lexer_file_buf_t *fb) {
+    close(fb->fd);
+    m_del_obj(mp_lexer_file_buf_t, fb);
+}
+
+mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
+    mp_lexer_file_buf_t *fb = m_new_obj(mp_lexer_file_buf_t);
+    fb->fd = open(filename, O_RDONLY);
+    if (fb->fd < 0) {
+        m_del_obj(mp_lexer_file_buf_t, fb);
+        return NULL;
+    }
+    int n = read(fb->fd, fb->buf, sizeof(fb->buf));
+    fb->len = n;
+    fb->pos = 0;
+    return mp_lexer_new(qstr_from_str(filename), fb, (mp_lexer_stream_next_char_t)file_buf_next_char, (mp_lexer_stream_close_t)file_buf_close);
 }
 
 #endif // MICROPY_ENABLE_LEXER_UNIX