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[Bug] grass.pygrass: Cats.delete(0) deletes all layer categories instead of category 0

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@Mohit-Ak is already working on this.

Since Oct 21, 2026.

  • #8095 by @Mohit-Ak — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
82/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c, python
Domain
data

Research direction

Start with Cats.delete() and CatsList.from_array() in the pygrass vector basic module (python/grass/pygrass/vector/basic.py in the GRASS repo). In delete, test cat is None instead of truthiness so that 0 goes through Vect_field_cat_del. In from_array, convert the input to a ctypes int array before the call. Done when the reproduction leaves [1, 2] after deleting 0, raises ValueError when category 0 is absent, and from_array([1, 2, 3]) populates the list.

Written by the indexing model from the issue text.

Description

Describe the bug

In grass.pygrass.vector.basic, Cats.delete() and CatsList.from_array() contain significant bugs:

1. Cats.delete(0) deletes all categories in the layer instead of category 0

In Cats.delete():

    def delete(self, cat=None, layer=1):
        if cat:
            self.n_del = libvect.Vect_field_cat_del(self.c_cats, layer, cat)
            err_msg = "Layer(%d)/category(%d) number does not exist" % (layer, cat)
        else:
            self.n_del = libvect.Vect_cat_del(self.c_cats, layer)
            err_msg = "Layer: %r does not exist" % layer
        if self.n_del == 0:
            raise ValueError(err_msg)

In GRASS GIS vector architecture, category numbers are non-negative integers (cat >= 0), and category 0 is a valid category number.
However, if cat: checks the truthiness of cat. When cat = 0:

  • bool(0) evaluates to False.
  • The code falls through to the else: branch and calls libvect.Vect_cat_del(self.c_cats, layer).
  • Vect_cat_del deletes all categories in the given layer rather than deleting category 0.

Furthermore, if category 0 is not present in the layer (for instance, the layer contains category 1), calling cats.delete(0, layer=1) should raise ValueError("Layer(1)/category(0) number does not exist"). Instead, it wipes out category 1 and returns successfully without raising any error, causing silent data loss.

2. CatsList.from_array() crashes with ctypes.ArgumentError

In CatsList.from_array():

    def from_array(self, array):
        # Vect_array_to_cat_list(const int *vals, int nvals, ***)
        # TODO: it's not working
        libvect.Vect_array_to_cat_list(array, len(array), self.c_cat_list)

libvect.Vect_array_to_cat_list requires a POINTER(c_int). Passing a Python list or array directly raises:

ctypes.ArgumentError: argument 1: TypeError: expected LP_c_long instance instead of list

To reproduce

from grass.pygrass.vector.basic import Cats, CatsList

# 1. Cats.delete(0) wipes out all categories
cats = Cats()
cats.set(0, layer=1)
cats.set(1, layer=1)
cats.set(2, layer=1)
print(cats.cat)  # [0, 1, 2]

# Attempt to delete only category 0:
cats.delete(0, layer=1)
print(cats.cat)  # [] -> All categories were deleted! Expected [1, 2].

# 2. CatsList.from_array raises ctypes.ArgumentError
cats_list = CatsList()
cats_list.from_array([1, 2, 3])
# ctypes.ArgumentError: argument 1: TypeError: expected LP_c_long instance instead of list

Expected behavior

  • cats.delete(0, layer=1) should only delete category 0 from layer 1 via Vect_field_cat_del, leaving other categories intact.
  • If category 0 does not exist in the layer, cats.delete(0, layer=1) should raise ValueError.
  • cats.delete(cat=None, layer=1) should continue to delete all categories in the layer when cat is None.
  • cats_list.from_array(array) should convert the integer iterable to a ctypes integer array and successfully populate cat_list.

System description

  • Operating System: All platforms (Linux, macOS, Windows)
  • GRASS version: main (development branch)
  • Python: 3.10+
Dominant language
C
Stars
1.2k
Forks
447
Avg merge
2d 7h
Merged PRs (30d)
108

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