diff --git a/src/paste3/io.py b/src/paste3/io.py
new file mode 100644
index 0000000..f4f1032
--- /dev/null
+++ b/src/paste3/io.py
@@ -0,0 +1,77 @@
+import scanpy as sc
+import numpy as np
+
+from pathlib import Path
+from collections import defaultdict
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+def process_files(g_fpath, s_fpath, w_fpath=None):
+    """Returns a list of AnnData objects."""
+
+    ext = Path(g_fpath[0]).suffix
+
+    if ext == ".csv":
+        if not (len(s_fpath) == len(g_fpath)):
+            ValueError("Length of spatial files doesn't equal number of gene files")
+        _slices = defaultdict()
+        for file in g_fpath:
+            # The header of this file is alphanumeric, so this file has to be imported as a string
+            _slices[get_shape(file)[0]] = sc.read_csv(file)
+
+        for file in s_fpath:
+            try:
+                _slice = _slices[get_shape(file)[0]]
+            except KeyError:
+                raise ValueError("Incomplete information for a slice")
+            else:
+                _slice.obsm["spatial"] = np.genfromtxt(
+                    file, delimiter=",", dtype="float64"
+                )
+
+        if w_fpath:
+            if not (len(w_fpath) == len(g_fpath)):
+                ValueError("Length of weight files doesn't equal number of gene files")
+            for file in w_fpath:
+                _slice = _slices[get_shape(file)[0]]
+                _slice.obsm["weights"] = np.genfromtxt(
+                    file, delimiter=",", dtype="float64"
+                )
+        else:
+            for k, v in _slices.items():
+                v.obsm["weights"] = np.ones((v.shape[0],)) / v.shape[0]
+
+        slices = list(_slices.values())
+    elif ext == ".h5ad":
+        slices = [sc.read_h5ad(file) for file in g_fpath]
+
+    else:
+        raise ValueError("Incorrect file type provided ")
+
+    return slices
+
+
+def get_shape(file_path):
+    """Determines the shapes of the csv without opening the files"""
+
+    def is_numeric(value):
+        try:
+            float(value)
+            return True
+        except ValueError:
+            return False
+
+    with open(file_path, "r") as file:
+        first_line = file.readline().strip()
+        num_columns = len(first_line.split(","))
+
+        num_rows = sum(1 for _ in file)
+
+        # Determine if the first row is a header
+        if all(is_numeric(val) for val in first_line.split(",")):
+            num_rows += 1
+
+    return num_rows, num_columns
diff --git a/tests/data/input/slice1.h5ad b/tests/data/input/slice1.h5ad
new file mode 100644
index 0000000..e2dd51b
Binary files /dev/null and b/tests/data/input/slice1.h5ad differ
diff --git a/tests/data/input/slice2.h5ad b/tests/data/input/slice2.h5ad
new file mode 100644
index 0000000..2260734
Binary files /dev/null and b/tests/data/input/slice2.h5ad differ
diff --git a/tests/data/input/slice3.h5ad b/tests/data/input/slice3.h5ad
new file mode 100644
index 0000000..10fc00c
Binary files /dev/null and b/tests/data/input/slice3.h5ad differ
diff --git a/tests/data/input/slice4.h5ad b/tests/data/input/slice4.h5ad
new file mode 100644
index 0000000..782ae31
Binary files /dev/null and b/tests/data/input/slice4.h5ad differ
diff --git a/tests/test_paste_cmd_line.py b/tests/test_paste_cmd_line.py
index ab05af2..823771e 100644
--- a/tests/test_paste_cmd_line.py
+++ b/tests/test_paste_cmd_line.py
@@ -1,8 +1,10 @@
 import pandas as pd
+import anndata as ad
 from pandas.testing import assert_frame_equal
 from pathlib import Path
 from collections import namedtuple
 from paste3.paste_cmd_line import main as paste_cmd_line
+from paste3.io import get_shape, process_files
 
 test_dir = Path(__file__).parent
 input_dir = test_dir / "data/input"
@@ -111,3 +113,38 @@ def test_cmd_line_pairwise(tmp_path):
         pd.read_csv(tmp_path / "paste_output/slice1_slice2_pairwise.csv"),
         pd.read_csv(output_dir / "slices_1_2_pairwise.csv"),
     )
+
+
+def test_process_files_csv():
+    """Ensure process files works with csv inputs."""
+    gene_fpath = []
+    spatial_fpath = []
+    for i in range(1, 5):
+        gene_fpath.append(Path(f"{input_dir}/slice{i}.csv"))
+        spatial_fpath.append(Path(f"{input_dir}/slice{i}_coor.csv"))
+
+    ad_objs = process_files(
+        gene_fpath,
+        spatial_fpath,
+    )
+    for obj in ad_objs:
+        assert isinstance(obj, ad.AnnData)
+
+
+def test_process_files_ann_data():
+    """Ensure process files works with Ann Data inputs."""
+    gene_fpath = []
+    for i in range(1, 5):
+        gene_fpath.append(Path(f"{input_dir}/slice{i}.h5ad"))
+
+    ad_objs = process_files(gene_fpath, s_fpath=None)
+    for obj in ad_objs:
+        assert isinstance(obj, ad.AnnData)
+
+
+def test_get_shape():
+    s_fpath = Path(f"{input_dir}/slice1.csv")
+    c_fpath = Path(f"{input_dir}/slice1_coor.csv")
+
+    assert get_shape(s_fpath) == (254, 7999)
+    assert get_shape(c_fpath) == (254, 2)