MAINT: numpy.i: Replace deprecated `sprintf` with `snprintf` by denproc · Pull Request #31056 · numpy/numpy

denproc · 2026-03-23T21:41:33Z

PR summary

This PR is for #31055.
The suggested changes in tools/swig/numpy.i replace deprecated sprintf with snprintf.

Similar issue was raised in #30997 and resolved in #30998.

First time committer introduction

Hi, I'm Denis Prokopenko, Researcher at King's College London. My main use of numpy is development of medical imaging methods. This particular PR was inspired by similar changes introduced in STIR UCL/STIR#1699.

AI Disclosure

No AI tools used

charris · 2026-03-27T02:29:42Z

@denproc AI does a nice job here. If you want to practice, that would be fine.

denproc · 2026-03-27T09:14:13Z

Hi @charris, I have addressed both snpintf, mentions in the original issue, and strcat, raised by @seberg, in the previsous commits.
I have also added additional validation of snprintf output to be extra safe as it could be negative or larger that buffer size.

Could you please let me know if there are any other outstanding issues and if I should also suggest megring it to maintenance branches for backport?

charris · 2026-03-27T15:14:37Z

@denproc I suggested AI because it produced cleaner, more readable code.

denproc · 2026-03-27T16:38:04Z

@charris, could you clarify what specifically could be improved for readability?
Is the concern mainly with the append logic, or would you prefer moving it into a helper function?

I initially tried to keep the changes minimal and local, but I’m happy to restructure it to reduce duplications if that’s preferred.

charris · 2026-03-27T17:07:26Z

I find the code here messy and overly verbose. Why not try AI yourself for suggestions?

denproc · 2026-03-27T17:22:50Z

@charris, I did try an AI-generated rewrite from scratch, but it seemed to keep the same inline snprintf/buffer bookkeeping structure, so I'm not sure it's actually cleaner than the current patch.
That’s why I asked for clarification, I assumed you had a more specific direction in mind for simplifying the current changes.

Codex suggestions:

int require_dimensions_n(PyArrayObject* ary,
                           int*           exact_dimensions,
                           int            n)
  {
    int success = 0;
    int i;
    int written;
    char dims_str[255] = "";
    size_t dims_len = 0;
    for (i = 0; i < n && !success; i++)
    {
      if (array_numdims(ary) == exact_dimensions[i])
      {
        success = 1;
      }
    }
    if (!success)
    {
      for (i = 0; i < n-1; i++)
      {
        size_t remaining = sizeof(dims_str) - dims_len;
        written = snprintf(dims_str + dims_len,
                           remaining,
                           "%d, ",
                           exact_dimensions[i]);
        if (written < 0)
        {
          break;
        }
        if ((size_t)written >= remaining)
        {
          dims_len = sizeof(dims_str) - 1;
          break;
        }
        dims_len += (size_t)written;
      }
      if (n > 0)
      {
        size_t remaining = sizeof(dims_str) - dims_len;
        written = snprintf(dims_str + dims_len,
                           remaining,
                           " or %d",
                           exact_dimensions[n-1]);
        if (written >= 0 && (size_t)written < remaining)
        {
          dims_len += (size_t)written;
        }
        else if (written >= 0)
        {
          dims_len = sizeof(dims_str) - 1;
        }
      }
      PyErr_Format(PyExc_TypeError,
                   "Array must have %s dimensions.  Given array has %d dimensions",
                   dims_str,
                   array_numdims(ary));
    }
    return success;
  }

  /* Require the given PyArrayObject to have a specified shape.  If the
   * array has the specified shape, return 1.  Otherwise, set the python
   * error string and return 0.
   */
  int require_size(PyArrayObject* ary,
                   npy_intp*      size,
                   int            n)
  {
    int i;
    int written;
    int success = 1;
    char desired_dims[255] = "[";
    size_t desired_len = 1;
    char actual_dims[255] = "[";
    size_t actual_len = 1;
    for(i=0; i < n;i++)
    {
      if (size[i] != -1 &&  size[i] != array_size(ary,i))
      {
        success = 0;
      }
    }
    if (!success)
    {
      for (i = 0; i < n; i++)
      {
        size_t remaining = sizeof(desired_dims) - desired_len;
        if (size[i] == -1)
        {
          written = snprintf(desired_dims + desired_len,
                             remaining,
                             "*,");
        }
        else
        {
          written = snprintf(desired_dims + desired_len,
                             remaining,
                             "%ld,",
                             (long int)size[i]);
        }
        if (written < 0)
        {
          break;
        }
        if ((size_t)written >= remaining)
        {
          desired_len = sizeof(desired_dims) - 1;
          break;
        }
        desired_len += (size_t)written;
      }
      if (desired_len > 1)
      {
        desired_dims[desired_len - 1] = ']';
      }
      else if (desired_len < sizeof(desired_dims) - 1)
      {
        desired_dims[desired_len++] = ']';
        desired_dims[desired_len] = '\0';
      }
      for (i = 0; i < n; i++)
      {
        size_t remaining = sizeof(actual_dims) - actual_len;
        written = snprintf(actual_dims + actual_len,
                           remaining,
                           "%ld,",
                           (long int)array_size(ary,i));
        if (written < 0)
        {
          break;
        }
        if ((size_t)written >= remaining)
        {
          actual_len = sizeof(actual_dims) - 1;
          break;
        }
        actual_len += (size_t)written;
      }
      if (actual_len > 1)
      {
        actual_dims[actual_len - 1] = ']';
      }
      else if (actual_len < sizeof(actual_dims) - 1)
      {
        actual_dims[actual_len++] = ']';
        actual_dims[actual_len] = '\0';
      }
      PyErr_Format(PyExc_TypeError,
                   "Array must have shape of %s.  Given array has shape of %s",
                   desired_dims,
                   actual_dims);
    }
    return success;
  }

charris · 2026-03-27T19:44:07Z

Note that the variable names don't clutter the code, keeps things on single lines, which makes it much easier to read. The difference I got using AI was similar:

diff --git a/tools/swig/numpy.i b/tools/swig/numpy.i
index abc1234..def5678 100644
--- a/tools/swig/numpy.i
+++ b/tools/swig/numpy.i
@@ -290,28 +290,40 @@ %fragment("NumPy_Array_Requirements",
    * number of dimensions. If the array has one of the specified number
    * of dimensions, return 1. Otherwise, set the python error string
    * and return 0.
    */
   int require_dimensions_n(PyArrayObject* ary,
                            int* exact_dimensions,
                            int n)
   {
     int success = 0;
     int i;
-    char dims_str[255] = "";
-    char s[255];
-    for (i = 0; i < n && !success; i++)
-    {
-      if (array_numdims(ary) == exact_dimensions[i])
-      {
-        success = 1;
-      }
-    }
-    if (!success)
-    {
-      for (i = 0; i < n-1; i++)
-      {
-        sprintf(s, "%d, ", exact_dimensions[i]);
-        strcat(dims_str,s);
-      }
-      sprintf(s, " or %d", exact_dimensions[n-1]);
-      strcat(dims_str,s);
+
+    /* Quick check */
+    for (i = 0; i < n; i++) {
+      if (array_numdims(ary) == exact_dimensions[i]) {
+        success = 1;
+        break;
+      }
+    }
+    if (success) return 1;
+
+    /* Build comma-separated list of dimensions safely with snprintf */
+    char dims_str[256] = "";
+    char *p = dims_str;
+    size_t remaining = sizeof(dims_str);
+
+    for (i = 0; i < n-1; i++) {
+      int written = snprintf(p, remaining, "%d, ", exact_dimensions[i]);
+      if (written < 0 || (size_t)written >= remaining) break;
+      p += written;
+      remaining -= (size_t)written;
+    }
+
+    /* Last dimension with "or " */
+    if (n > 0) {
+      snprintf(p, remaining, "or %d", exact_dimensions[n-1]);
+    }
+
       PyErr_Format(PyExc_TypeError,
                    "Array must have %s dimensions. Given array has %d dimensions",
                    dims_str,
                    array_numdims(ary));
     }
     return success;
   }
@@ -320,32 +332,38 @@ %fragment("NumPy_Array_Requirements",
    * array has the specified shape, return 1. Otherwise, set the python
    * error string and return 0.
    */
   int require_size(PyArrayObject* ary,
                    npy_intp* size,
                    int n)
   {
     int i;
     int success = 1;
-    size_t len;
-    char desired_dims[255] = "[";
-    char s[255];
-    char actual_dims[255] = "[";
+
     for(i=0; i < n; i++)
     {
       if (size[i] != -1 && size[i] != array_size(ary,i))
       {
         success = 0;
+        break;
       }
     }
-    if (!success)
-    {
-      for (i = 0; i < n; i++)
-      {
-        if (size[i] == -1)
-        {
-          sprintf(s, "*,");
-        }
-        else
-        {
-          sprintf(s, "%ld,", (long int)size[i]);
-        }
-        strcat(desired_dims,s);
-      }
-      len = strlen(desired_dims);
-      desired_dims[len-1] = ']';
-      for (i = 0; i < n; i++)
-      {
-        sprintf(s, "%ld,", (long int)array_size(ary,i));
-        strcat(actual_dims,s);
-      }
-      len = strlen(actual_dims);
-      actual_dims[len-1] = ']';
+
+    if (success) return 1;
+
+    /* Build desired and actual shape strings safely */
+    char desired_dims[256] = "[";
+    char actual_dims[256] = "[";
+    char s[64];
+
+    for (i = 0; i < n; i++)
+    {
+      if (size[i] == -1)
+      {
+        strncat(desired_dims, "*,", sizeof(desired_dims) - strlen(desired_dims) - 1);
+      }
+      else
+      {
+        snprintf(s, sizeof(s), "%ld,", (long int)size[i]);
+        strncat(desired_dims, s, sizeof(desired_dims) - strlen(desired_dims) - 1);
+      }
+
+      snprintf(s, sizeof(s), "%ld,", (long int)array_size(ary, i));
+      strncat(actual_dims, s, sizeof(actual_dims) - strlen(actual_dims) - 1);
+    }
+
+    /* Replace trailing comma with ']' */
+    size_t len = strlen(desired_dims);
+    if (len > 0) desired_dims[len-1] = ']';
+
+    len = strlen(actual_dims);
+    if (len > 0) actual_dims[len-1] = ']';
+
       PyErr_Format(PyExc_TypeError,
                    "Array must have shape of %s. Given array has shape of %s",
                    desired_dims,
                    actual_dims);
     }
     return success;
   }

I haven't checked it for correctness, I just wanted to see what it looked like. We should probably have #include <strings.h> for strncat if we keep it -- I note that you don't. It currently comes in indirectly in Python.h, but direct inclusion is recommended.

denproc · 2026-03-27T21:57:51Z

@charris thanks! To address the deprecation issue, I updated the code and replaced sprintf/strcat with snprintf/strncat, while keeping the changes minimal and clean, instead of replacing everything with just snprintf.

There are still potential improvements (avoiding repeated strlen calls and making truncation handling more explicit, which may add some verbosity), but I'm not sure if it is preferable to handle those in a separate PR. What do you think?

charris · 2026-03-28T17:08:16Z

Thanks @denproc . That was much easier to review for correctness with the minimal changes. The AI was doing some extra optimizations, but I doubt is matters here.

charris · 2026-03-28T17:33:54Z

Note that the hot paths in both functions involved are already taken care of, speed in the error paths doesn't matter much.

charris · 2026-03-28T18:26:28Z

The only remaining useful improvements I see in the AI diff are the comments, and the break in the hot loop. But not really worth addressing unless someone complains :)

…1056)

MAINT: numpy.i: Replace deprecated `sprintf` with `snprintf` (#31056)

fix-sprintf-deprecation

dc31d52

denproc changed the title ~~MAINT: numpy.i: Replace deprecated 'sprintf' with snprintf~~ MAINT: numpy.i: Replace deprecated sprintf with snprintf Mar 23, 2026

jorenham added the 03 - Maintenance label Mar 23, 2026

jorenham linked an issue Mar 23, 2026 that may be closed by this pull request

MAINT: numpy.i: 'sprintf' is deprecated #31055

Closed

charris added the 09 - Backport-Candidate PRs tagged should be backported label Mar 24, 2026

seberg reviewed Mar 24, 2026

View reviewed changes

Comment thread tools/swig/numpy.i Outdated

denproc added 2 commits March 24, 2026 15:05

replace strcat use with snprintf

0f51fe5

formatting

bfa9b51

denproc mentioned this pull request Mar 24, 2026

Fix buffer size issue UCL/STIR#1699

Merged

6 tasks

denproc requested a review from seberg March 27, 2026 09:15

replaced deprecarted methods with snprintf and strncat

914c752

charris merged commit e13e931 into numpy:main Mar 28, 2026
78 checks passed

charris pushed a commit to charris/numpy that referenced this pull request Mar 28, 2026

MAINT: numpy.i: Replace deprecated sprintf with snprintf (numpy#3…

fa74b2d

…1056)

charris mentioned this pull request Mar 28, 2026

MAINT: numpy.i: Replace deprecated sprintf with snprintf (#31056) #31084

Merged

charris added a commit that referenced this pull request Mar 28, 2026

Merge pull request #31084 from charris/backport-31056

02e838b

MAINT: numpy.i: Replace deprecated `sprintf` with `snprintf` (#31056)

denproc deleted the fix_sprintf_deprecation branch March 28, 2026 23:34

charris removed the 09 - Backport-Candidate PRs tagged should be backported label Apr 10, 2026

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

MAINT: numpy.i: Replace deprecated `sprintf` with `snprintf`#31056

MAINT: numpy.i: Replace deprecated `sprintf` with `snprintf`#31056
charris merged 4 commits into
numpy:mainfrom
denproc:fix_sprintf_deprecation

denproc commented Mar 23, 2026 •

edited

Loading

Uh oh!

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026 •

edited

Loading

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026 •

edited

Loading

Uh oh!

Uh oh!

charris commented Mar 28, 2026

Uh oh!

charris commented Mar 28, 2026

Uh oh!

charris commented Mar 28, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Uh oh!

Conversation

denproc commented Mar 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

PR summary

First time committer introduction

AI Disclosure

Uh oh!

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026

Uh oh!

charris commented Mar 27, 2026

Uh oh!

denproc commented Mar 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

charris commented Mar 28, 2026

Uh oh!

charris commented Mar 28, 2026

Uh oh!

charris commented Mar 28, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

denproc commented Mar 23, 2026 •

edited

Loading

denproc commented Mar 27, 2026 •

edited

Loading

denproc commented Mar 27, 2026 •

edited

Loading