Improve exception traceback readability

.pre-commit-config.yaml

@@ -35,7 +35,8 @@ repos:
     rev: 6.0.0
     hooks:
       - id: flake8
-        additional_dependencies: [flake8-2020, flake8-implicit-str-concat]
+        additional_dependencies:
+          [flake8-2020, flake8-errmsg, flake8-implicit-str-concat]
 
   - repo: https://github.com/pre-commit/pygrep-hooks
     rev: v1.9.0

docs/example/DdsImagePlugin.py

@@ -211,13 +211,16 @@ class DdsImageFile(ImageFile.ImageFile):
 
     def _open(self):
         if not _accept(self.fp.read(4)):
-            raise SyntaxError("not a DDS file")
+            msg = "not a DDS file"
+            raise SyntaxError(msg)
         (header_size,) = struct.unpack("<I", self.fp.read(4))
         if header_size != 124:
-            raise OSError(f"Unsupported header size {repr(header_size)}")
+            msg = f"Unsupported header size {repr(header_size)}"
+            raise OSError(msg)
         header_bytes = self.fp.read(header_size - 4)
         if len(header_bytes) != 120:
-            raise OSError(f"Incomplete header: {len(header_bytes)} bytes")
+            msg = f"Incomplete header: {len(header_bytes)} bytes"
+            raise OSError(msg)
         header = BytesIO(header_bytes)
 
         flags, height, width = struct.unpack("<3I", header.read(12))
@@ -237,7 +240,8 @@ def _open(self):
         elif fourcc == b"DXT5":
             self.decoder = "DXT5"
         else:
-            raise NotImplementedError(f"Unimplemented pixel format {fourcc}")
+            msg = f"Unimplemented pixel format {fourcc}"
+            raise NotImplementedError(msg)
 
         self.tile = [(self.decoder, (0, 0) + self.size, 0, (self.mode, 0, 1))]
 
@@ -252,7 +256,8 @@ def decode(self, buffer):
         try:
             self.set_as_raw(_dxt1(self.fd, self.state.xsize, self.state.ysize))
         except struct.error as e:
-            raise OSError("Truncated DDS file") from e
+            msg = "Truncated DDS file"
+            raise OSError(msg) from e
         return -1, 0
 
 
@@ -263,7 +268,8 @@ def decode(self, buffer):
         try:
             self.set_as_raw(_dxt5(self.fd, self.state.xsize, self.state.ysize))
         except struct.error as e:
-            raise OSError("Truncated DDS file") from e
+            msg = "Truncated DDS file"
+            raise OSError(msg) from e
         return -1, 0
 
 

docs/handbook/writing-your-own-image-plugin.rst

@@ -78,7 +78,8 @@ true color.
             elif bits == 24:
                 self.mode = "RGB"
             else:
-                raise SyntaxError("unknown number of bits")
+                msg = "unknown number of bits"
+                raise SyntaxError(msg)
 
             # data descriptor
             self.tile = [("raw", (0, 0) + self.size, 128, (self.mode, 0, 1))]

setup.py

@@ -362,15 +362,15 @@ def finalize_options(self):
                 self.feature.required.discard(x)
                 _dbg("Disabling %s", x)
                 if getattr(self, f"enable_{x}"):
-                    raise ValueError(
-                        f"Conflicting options: --enable-{x} and --disable-{x}"
-                    )
+                    msg = f"Conflicting options: --enable-{x} and --disable-{x}"
+                    raise ValueError(msg)
                 if x == "freetype":
                     _dbg("--disable-freetype implies --disable-raqm")
                     if getattr(self, "enable_raqm"):
-                        raise ValueError(
+                        msg = (
                             "Conflicting options: --enable-raqm and --disable-freetype"
                         )
+                        raise ValueError(msg)
                     setattr(self, "disable_raqm", True)
             if getattr(self, f"enable_{x}"):
                 _dbg("Requiring %s", x)
@@ -381,13 +381,11 @@ def finalize_options(self):
         for x in ("raqm", "fribidi"):
             if getattr(self, f"vendor_{x}"):
                 if getattr(self, "disable_raqm"):
-                    raise ValueError(
-                        f"Conflicting options: --vendor-{x} and --disable-raqm"
-                    )
+                    msg = f"Conflicting options: --vendor-{x} and --disable-raqm"
+                    raise ValueError(msg)
                 if x == "fribidi" and not getattr(self, "vendor_raqm"):
-                    raise ValueError(
-                        f"Conflicting options: --vendor-{x} and not --vendor-raqm"
-                    )
+                    msg = f"Conflicting options: --vendor-{x} and not --vendor-raqm"
+                    raise ValueError(msg)
                 _dbg("Using vendored version of %s", x)
                 self.feature.vendor.add(x)
 

src/PIL/BdfFontFile.py

@@ -86,7 +86,8 @@ def __init__(self, fp):
 
         s = fp.readline()
         if s[:13] != b"STARTFONT 2.1":
-            raise SyntaxError("not a valid BDF file")
+            msg = "not a valid BDF file"
+            raise SyntaxError(msg)
 
         props = {}
         comments = []

src/PIL/BlpImagePlugin.py

@@ -65,7 +65,8 @@ def __getattr__(name):
             if name in enum.__members__:
                 deprecate(f"{prefix}{name}", 10, f"{enum.__name__}.{name}")
                 return enum[name]
-    raise AttributeError(f"module '{__name__}' has no attribute '{name}'")
+    msg = f"module '{__name__}' has no attribute '{name}'"
+    raise AttributeError(msg)
 
 
 def unpack_565(i):
@@ -278,7 +279,8 @@ def _open(self):
         if self.magic in (b"BLP1", b"BLP2"):
             decoder = self.magic.decode()
         else:
-            raise BLPFormatError(f"Bad BLP magic {repr(self.magic)}")
+            msg = f"Bad BLP magic {repr(self.magic)}"
+            raise BLPFormatError(msg)
 
         self.mode = "RGBA" if self._blp_alpha_depth else "RGB"
         self.tile = [(decoder, (0, 0) + self.size, 0, (self.mode, 0, 1))]
@@ -292,7 +294,8 @@ def decode(self, buffer):
             self._read_blp_header()
             self._load()
         except struct.error as e:
-            raise OSError("Truncated BLP file") from e
+            msg = "Truncated BLP file"
+            raise OSError(msg) from e
         return -1, 0
 
     def _read_blp_header(self):
@@ -354,13 +357,11 @@ def _load(self):
                 data = self._read_bgra(palette)
                 self.set_as_raw(bytes(data))
             else:
-                raise BLPFormatError(
-                    f"Unsupported BLP encoding {repr(self._blp_encoding)}"
-                )
+                msg = f"Unsupported BLP encoding {repr(self._blp_encoding)}"
+                raise BLPFormatError(msg)
         else:
-            raise BLPFormatError(
-                f"Unsupported BLP compression {repr(self._blp_encoding)}"
-            )
+            msg = f"Unsupported BLP compression {repr(self._blp_encoding)}"
+            raise BLPFormatError(msg)
 
     def _decode_jpeg_stream(self):
         from .JpegImagePlugin import JpegImageFile
@@ -415,16 +416,15 @@ def _load(self):
                         for d in decode_dxt5(self._safe_read(linesize)):
                             data += d
                 else:
-                    raise BLPFormatError(
-                        f"Unsupported alpha encoding {repr(self._blp_alpha_encoding)}"
-                    )
+                    msg = f"Unsupported alpha encoding {repr(self._blp_alpha_encoding)}"
+                    raise BLPFormatError(msg)
             else:
-                raise BLPFormatError(f"Unknown BLP encoding {repr(self._blp_encoding)}")
+                msg = f"Unknown BLP encoding {repr(self._blp_encoding)}"
+                raise BLPFormatError(msg)
 
         else:
-            raise BLPFormatError(
-                f"Unknown BLP compression {repr(self._blp_compression)}"
-            )
+            msg = f"Unknown BLP compression {repr(self._blp_compression)}"
+            raise BLPFormatError(msg)
 
         self.set_as_raw(bytes(data))
 
@@ -460,7 +460,8 @@ def encode(self, bufsize):
 
 def _save(im, fp, filename, save_all=False):
     if im.mode != "P":
-        raise ValueError("Unsupported BLP image mode")
+        msg = "Unsupported BLP image mode"
+        raise ValueError(msg)
 
     magic = b"BLP1" if im.encoderinfo.get("blp_version") == "BLP1" else b"BLP2"
     fp.write(magic)

src/PIL/BmpImagePlugin.py

@@ -146,7 +146,8 @@ def _bitmap(self, header=0, offset=0):
                     file_info["a_mask"],
                 )
         else:
-            raise OSError(f"Unsupported BMP header type ({file_info['header_size']})")
+            msg = f"Unsupported BMP header type ({file_info['header_size']})"
+            raise OSError(msg)
 
         # ------------------ Special case : header is reported 40, which
         # ---------------------- is shorter than real size for bpp >= 16
@@ -164,7 +165,8 @@ def _bitmap(self, header=0, offset=0):
         # ---------------------- Check bit depth for unusual unsupported values
         self.mode, raw_mode = BIT2MODE.get(file_info["bits"], (None, None))
         if self.mode is None:
-            raise OSError(f"Unsupported BMP pixel depth ({file_info['bits']})")
+            msg = f"Unsupported BMP pixel depth ({file_info['bits']})"
+            raise OSError(msg)
 
         # ---------------- Process BMP with Bitfields compression (not palette)
         decoder_name = "raw"
@@ -205,23 +207,27 @@ def _bitmap(self, header=0, offset=0):
                 ):
                     raw_mode = MASK_MODES[(file_info["bits"], file_info["rgb_mask"])]
                 else:
-                    raise OSError("Unsupported BMP bitfields layout")
+                    msg = "Unsupported BMP bitfields layout"
+                    raise OSError(msg)
             else:
-                raise OSError("Unsupported BMP bitfields layout")
+                msg = "Unsupported BMP bitfields layout"
+                raise OSError(msg)
         elif file_info["compression"] == self.RAW:
             if file_info["bits"] == 32 and header == 22:  # 32-bit .cur offset
                 raw_mode, self.mode = "BGRA", "RGBA"
         elif file_info["compression"] in (self.RLE8, self.RLE4):
             decoder_name = "bmp_rle"
         else:
-            raise OSError(f"Unsupported BMP compression ({file_info['compression']})")
+            msg = f"Unsupported BMP compression ({file_info['compression']})"
+            raise OSError(msg)
 
         # --------------- Once the header is processed, process the palette/LUT
         if self.mode == "P":  # Paletted for 1, 4 and 8 bit images
 
             # ---------------------------------------------------- 1-bit images
             if not (0 < file_info["colors"] <= 65536):
-                raise OSError(f"Unsupported BMP Palette size ({file_info['colors']})")
+                msg = f"Unsupported BMP Palette size ({file_info['colors']})"
+                raise OSError(msg)
             else:
                 padding = file_info["palette_padding"]
                 palette = read(padding * file_info["colors"])
@@ -271,7 +277,8 @@ def _open(self):
         head_data = self.fp.read(14)
         # choke if the file does not have the required magic bytes
         if not _accept(head_data):
-            raise SyntaxError("Not a BMP file")
+            msg = "Not a BMP file"
+            raise SyntaxError(msg)
         # read the start position of the BMP image data (u32)
         offset = i32(head_data, 10)
         # load bitmap information (offset=raster info)
@@ -383,7 +390,8 @@ def _save(im, fp, filename, bitmap_header=True):
     try:
         rawmode, bits, colors = SAVE[im.mode]
     except KeyError as e:
-        raise OSError(f"cannot write mode {im.mode} as BMP") from e
+        msg = f"cannot write mode {im.mode} as BMP"
+        raise OSError(msg) from e
 
     info = im.encoderinfo
 
@@ -411,7 +419,8 @@ def _save(im, fp, filename, bitmap_header=True):
         offset = 14 + header + colors * 4
         file_size = offset + image
         if file_size > 2**32 - 1:
-            raise ValueError("File size is too large for the BMP format")
+            msg = "File size is too large for the BMP format"
+            raise ValueError(msg)
         fp.write(
             b"BM"  # file type (magic)
             + o32(file_size)  # file size

src/PIL/BufrStubImagePlugin.py

@@ -42,7 +42,8 @@ def _open(self):
         offset = self.fp.tell()
 
         if not _accept(self.fp.read(4)):
-            raise SyntaxError("Not a BUFR file")
+            msg = "Not a BUFR file"
+            raise SyntaxError(msg)
 
         self.fp.seek(offset)
 
@@ -60,7 +61,8 @@ def _load(self):
 
 def _save(im, fp, filename):
     if _handler is None or not hasattr(_handler, "save"):
-        raise OSError("BUFR save handler not installed")
+        msg = "BUFR save handler not installed"
+        raise OSError(msg)
     _handler.save(im, fp, filename)
 
 

src/PIL/CurImagePlugin.py

@@ -43,7 +43,8 @@ def _open(self):
         # check magic
         s = self.fp.read(6)
         if not _accept(s):
-            raise SyntaxError("not a CUR file")
+            msg = "not a CUR file"
+            raise SyntaxError(msg)
 
         # pick the largest cursor in the file
         m = b""
@@ -54,7 +55,8 @@ def _open(self):
             elif s[0] > m[0] and s[1] > m[1]:
                 m = s
         if not m:
-            raise TypeError("No cursors were found")
+            msg = "No cursors were found"
+            raise TypeError(msg)
 
         # load as bitmap
         self._bitmap(i32(m, 12) + offset)

src/PIL/DcxImagePlugin.py

@@ -47,7 +47,8 @@ def _open(self):
         # Header
         s = self.fp.read(4)
         if not _accept(s):
-            raise SyntaxError("not a DCX file")
+            msg = "not a DCX file"
+            raise SyntaxError(msg)
 
         # Component directory
         self._offset = []

src/PIL/DdsImagePlugin.py

@@ -114,13 +114,16 @@ class DdsImageFile(ImageFile.ImageFile):
 
     def _open(self):
         if not _accept(self.fp.read(4)):
-            raise SyntaxError("not a DDS file")
+            msg = "not a DDS file"
+            raise SyntaxError(msg)
         (header_size,) = struct.unpack("<I", self.fp.read(4))
         if header_size != 124:
-            raise OSError(f"Unsupported header size {repr(header_size)}")
+            msg = f"Unsupported header size {repr(header_size)}"
+            raise OSError(msg)
         header_bytes = self.fp.read(header_size - 4)
         if len(header_bytes) != 120:
-            raise OSError(f"Incomplete header: {len(header_bytes)} bytes")
+            msg = f"Incomplete header: {len(header_bytes)} bytes"
+            raise OSError(msg)
         header = BytesIO(header_bytes)
 
         flags, height, width = struct.unpack("<3I", header.read(12))
@@ -216,11 +219,11 @@ def _open(self):
                         self.info["gamma"] = 1 / 2.2
                     return
                 else:
-                    raise NotImplementedError(
-                        f"Unimplemented DXGI format {dxgi_format}"
-                    )
+                    msg = f"Unimplemented DXGI format {dxgi_format}"
+                    raise NotImplementedError(msg)
             else:
-                raise NotImplementedError(f"Unimplemented pixel format {repr(fourcc)}")
+                msg = f"Unimplemented pixel format {repr(fourcc)}"
+                raise NotImplementedError(msg)
 
             self.tile = [
                 ("bcn", (0, 0) + self.size, data_start, (n, self.pixel_format))
@@ -232,7 +235,8 @@ def load_seek(self, pos):
 
 def _save(im, fp, filename):
     if im.mode not in ("RGB", "RGBA", "L", "LA"):
-        raise OSError(f"cannot write mode {im.mode} as DDS")
+        msg = f"cannot write mode {im.mode} as DDS"
+        raise OSError(msg)
 
     rawmode = im.mode
     masks = [0xFF0000, 0xFF00, 0xFF]