apacheGH-39780: [Python][Parquet] Support hashing for FileMetaData an…

…d ParquetSchema (apache#39781)

I think the hash, especially for `FileMetaData` could be better, maybe just use return of `__repr__`, even though that won't include row group info?

### Rationale for this change

Helpful for dependent projects. 

### What changes are included in this PR?

Impl `__hash__` for `ParquetSchema` and `FileMetaData`

### Are these changes tested?

Yes

### Are there any user-facing changes?

Supports hashing metadata:

```python
In [1]: import pyarrow.parquet as pq

In [2]: f = pq.ParquetFile('test.parquet')

In [3]: hash(f.metadata)
Out[3]: 4816453453708427907

In [4]: hash(f.metadata.schema)
Out[4]: 2300988959078172540
```
* Closes: apache#39780

Authored-by: Miles Granger <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

python/pyarrow/_parquet.pyx

@@ -849,6 +849,13 @@ cdef class FileMetaData(_Weakrefable):
         cdef Buffer buffer = sink.getvalue()
         return _reconstruct_filemetadata, (buffer,)
 
+    def __hash__(self):
+        return hash((self.schema,
+                     self.num_rows,
+                     self.num_row_groups,
+                     self.format_version,
+                     self.serialized_size))
+
     def __repr__(self):
         return """{0}
   created_by: {1}
@@ -1071,6 +1078,9 @@ cdef class ParquetSchema(_Weakrefable):
     def __getitem__(self, i):
         return self.column(i)
 
+    def __hash__(self):
+        return hash(self.schema.ToString())
+
     @property
     def names(self):
         """Name of each field (list of str)."""

python/pyarrow/tests/parquet/test_metadata.py

@@ -499,6 +499,32 @@ def test_multi_dataset_metadata(tempdir):
     assert md['serialized_size'] > 0
 
 
+def test_metadata_hashing(tempdir):
+    path1 = str(tempdir / "metadata1")
+    schema1 = pa.schema([("a", "int64"), ("b", "float64")])
+    pq.write_metadata(schema1, path1)
+    parquet_meta1 = pq.read_metadata(path1)
+
+    # Same as 1, just different path
+    path2 = str(tempdir / "metadata2")
+    schema2 = pa.schema([("a", "int64"), ("b", "float64")])
+    pq.write_metadata(schema2, path2)
+    parquet_meta2 = pq.read_metadata(path2)
+
+    # different schema
+    path3 = str(tempdir / "metadata3")
+    schema3 = pa.schema([("a", "int64"), ("b", "float32")])
+    pq.write_metadata(schema3, path3)
+    parquet_meta3 = pq.read_metadata(path3)
+
+    # Deterministic
+    assert hash(parquet_meta1) == hash(parquet_meta1)  # equal w/ same instance
+    assert hash(parquet_meta1) == hash(parquet_meta2)  # equal w/ different instance
+
+    # Not the same as other metadata with different schema
+    assert hash(parquet_meta1) != hash(parquet_meta3)
+
+
 @pytest.mark.filterwarnings("ignore:Parquet format:FutureWarning")
 def test_write_metadata(tempdir):
     path = str(tempdir / "metadata")