diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 474b5d2..517a4a9 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -2,77 +2,68 @@ ci:
   autoupdate_commit_msg: "chore: update pre-commit hooks"
   autofix_commit_msg: "style: pre-commit fixes"
 
-repos:
-  - repo: https://github.com/psf/black-pre-commit-mirror
-    rev: "23.7.0"
-    hooks:
-      - id: black-jupyter
 
-  - repo: https://github.com/adamchainz/blacken-docs
-    rev: "1.16.0"
-    hooks:
-      - id: blacken-docs
-        additional_dependencies: [black==23.7.0]
+repos:
+- repo: https://github.com/pre-commit/pre-commit-hooks
+  rev: v4.4.0
+  hooks:
+  - id: check-added-large-files
+  - id: check-case-conflict
+  - id: check-merge-conflict
+  - id: check-symlinks
+  - id: check-yaml
+  - id: debug-statements
+  - id: end-of-file-fixer
+    exclude_types: [svg]
+  - id: mixed-line-ending
+  - id: requirements-txt-fixer
+  - id: trailing-whitespace
+  - id: name-tests-test
+    args: ["--pytest-test-first"]
 
-  - repo: https://github.com/pre-commit/pre-commit-hooks
-    rev: "v4.4.0"
-    hooks:
-      - id: check-added-large-files
-      - id: check-case-conflict
-      - id: check-merge-conflict
-      - id: check-symlinks
-      - id: check-yaml
-      - id: debug-statements
-      - id: end-of-file-fixer
-      - id: mixed-line-ending
-      - id: name-tests-test
-        args: ["--pytest-test-first"]
-      - id: requirements-txt-fixer
-      - id: trailing-whitespace
+- repo: https://github.com/asottile/setup-cfg-fmt
+  rev: v2.4.0
+  hooks:
+  - id: setup-cfg-fmt
+    args: [--include-version-classifiers, --max-py-version=3.11]
 
-  - repo: https://github.com/pre-commit/pygrep-hooks
-    rev: "v1.10.0"
-    hooks:
-      - id: rst-backticks
-      - id: rst-directive-colons
-      - id: rst-inline-touching-normal
+- repo: https://github.com/psf/black-pre-commit-mirror
+  rev: 23.9.1
+  hooks:
+  - id: black
 
-  - repo: https://github.com/pre-commit/mirrors-prettier
-    rev: "v3.0.3"
-    hooks:
-      - id: prettier
-        types_or: [yaml, markdown, html, css, scss, javascript, json]
-        args: [--prose-wrap=always]
+- repo: https://github.com/cheshirekow/cmake-format-precommit
+  rev: v0.6.13
+  hooks:
+  - id: cmake-format
+    additional_dependencies: [pyyaml]
 
-  - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: "v0.0.287"
-    hooks:
-      - id: ruff
-        args: ["--fix", "--show-fixes"]
+- repo: https://github.com/astral-sh/ruff-pre-commit
+  rev: v0.0.291
+  hooks:
+  - id: ruff
+    args: ["--fix", "--show-fixes"]
 
-  - repo: https://github.com/pre-commit/mirrors-mypy
-    rev: "v1.5.1"
-    hooks:
-      - id: mypy
-        files: src|tests
-        args: []
-        additional_dependencies:
-          - pytest
+- repo: https://github.com/codespell-project/codespell
+  rev: v2.2.5
+  hooks:
+  - id: codespell
+    args: ["-L", "ue,subjet,parms,fo,numer,thre,gaus"]
 
-  - repo: https://github.com/codespell-project/codespell
-    rev: "v2.2.5"
-    hooks:
-      - id: codespell
+- repo: local
+  hooks:
+  - id: disallow-caps
+    name: disallow improper capitalization
+    language: pygrep
+    entry: PyBind|Cmake|CCache|Github|PyTest
+    exclude: .pre-commit-config.yaml
 
-  - repo: https://github.com/shellcheck-py/shellcheck-py
-    rev: "v0.9.0.5"
-    hooks:
-      - id: shellcheck
+- repo: https://github.com/shellcheck-py/shellcheck-py
+  rev: "v0.9.0.5"
+  hooks:
+  - id: shellcheck
 
-  - repo: local
-    hooks:
-      - id: disallow-caps
-        name: Disallow improper capitalization
-        language: pygrep
-        entry: PyBind|Numpy|Cmake|CCache|Github|PyTest
-        exclude: .pre-commit-config.yaml
+- repo: https://github.com/asottile/pyupgrade
+  rev: v3.13.0
+  hooks:
+  - id: pyupgrade
diff --git a/README.md b/README.md
index 7d7f3b2..427a85f 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# Odapt
+# odapt
 
 [![Actions Status][actions-badge]][actions-link]
 [![Documentation Status][rtd-badge]][rtd-link]
diff --git a/docs/conf.py b/docs/conf.py
index 6316841..6e7644e 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -2,7 +2,7 @@
 
 import importlib.metadata
 
-project = "Odapt"
+project = "odapt"
 copyright = "2023, Zoë Bilodeau"
 author = "Zoë Bilodeau"
 version = release = importlib.metadata.version("odapt")
diff --git a/docs/index.md b/docs/index.md
index f926b6f..18cdf68 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -1,4 +1,4 @@
-# Odapt
+# odapt
 
 ```{toctree}
 :maxdepth: 2
diff --git a/pyproject.toml b/pyproject.toml
index 5edf88b..dcaa5e5 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -29,7 +29,7 @@ classifiers = [
   "Typing :: Typed",
 ]
 dynamic = ["version"]
-dependencies = []
+dependencies = ["numpy>=1.18.0", "uproot>=5.0.0"]
 
 [project.optional-dependencies]
 test = [
@@ -49,10 +49,10 @@ docs = [
 ]
 
 [project.urls]
-Homepage = "https://github.com/zbilodea/Odapt"
-"Bug Tracker" = "https://github.com/zbilodea/Odapt/issues"
-Discussions = "https://github.com/zbilodea/Odapt/discussions"
-Changelog = "https://github.com/zbilodea/Odapt/releases"
+Homepage = "https://github.com/zbilodea/odapt"
+"Bug Tracker" = "https://github.com/zbilodea/odapt/issues"
+Discussions = "https://github.com/zbilodea/odapt/discussions"
+Changelog = "https://github.com/zbilodea/odapt/releases"
 
 
 [tool.hatch]
@@ -160,3 +160,6 @@ messages_control.disable = [
   "missing-module-docstring",
   "wrong-import-position",
 ]
+
+[project.scripts]
+odapt = "odapt.src.operations.hadd:main"
diff --git a/src/odapt/__init__.py b/src/odapt/__init__.py
index 872d7dc..5e1880d 100644
--- a/src/odapt/__init__.py
+++ b/src/odapt/__init__.py
@@ -1,10 +1,11 @@
 """
 Copyright (c) 2023 Zoë Bilodeau. All rights reserved.
 
-Odapt: File conversion package.
+odapt: File conversion package.
 """
 from __future__ import annotations
 
 from odapt._version import version as __version__
+from odapt.operations import hadd  # noqa: F401
 
 __all__ = ["__version__"]
diff --git a/src/odapt/operations/__init__.py b/src/odapt/operations/__init__.py
new file mode 100644
index 0000000..d477324
--- /dev/null
+++ b/src/odapt/operations/__init__.py
@@ -0,0 +1,3 @@
+from __future__ import annotations
+
+from odapt.operations.hadd import hadd, main  # noqa: F401
diff --git a/src/odapt/operations/hadd.py b/src/odapt/operations/hadd.py
new file mode 100644
index 0000000..f2bb792
--- /dev/null
+++ b/src/odapt/operations/hadd.py
@@ -0,0 +1,478 @@
+from __future__ import annotations
+
+import argparse
+from pathlib import Path
+
+import numpy as np
+import uproot
+
+
+def hadd_1d(destination, file, key, first):
+    """
+    Args:
+    destination (path-like): Name of the output file or file path.
+    file (ReadOnlyDirectory): ROOT file to read histogram from.
+    key (str): key to reference histogram to be added.
+    first (bool): if True, special case for first of a certain histogram
+        to be added to the new file.
+    """
+    outfile = uproot.open(destination)
+    try:
+        hist = file[key]
+    except ValueError:
+        msg = "Key missing from {file}"
+        raise ValueError(msg) from None
+    if first:
+        member_data = np.array(
+            [
+                hist.member("fEntries"),
+                hist.member("fTsumw"),
+                hist.member("fTsumw2"),
+                hist.member("fTsumwx"),
+                hist.member("fTsumwx2"),
+            ]
+        )
+        return uproot.writing.identify.to_TH1x(
+            hist.member("fName"),
+            hist.member("fTitle"),
+            hist.values(flow=True),
+            *member_data,
+            hist.variances(flow=True),
+            hist.member("fXaxis"),
+        )
+    if hist.member("fN") == outfile[key].member("fN"):
+        member_data = np.array(
+            [
+                hist.member("fEntries"),
+                hist.member("fTsumw"),
+                hist.member("fTsumw2"),
+                hist.member("fTsumwx"),
+                hist.member("fTsumwx2"),
+            ]
+        )
+        h_sum = uproot.writing.identify.to_TH1x(
+            hist.member("fName"),
+            hist.member("fTitle"),
+            outfile[key].values(flow=True) + hist.values(flow=True),
+            *np.add(
+                np.array(
+                    [
+                        outfile[key].member("fEntries"),
+                        outfile[key].member("fTsumw"),
+                        outfile[key].member("fTsumw2"),
+                        outfile[key].member("fTsumwx"),
+                        outfile[key].member("fTsumwx2"),
+                    ]
+                ),
+                member_data,
+            ),
+            outfile[key].variances(flow=True) + hist.variances(flow=True),
+            hist.member("fXaxis"),
+        )
+        outfile.close()
+        return h_sum
+
+    msg = "Bins must be the same for histograms to be added, not "
+    raise ValueError(
+        msg,
+        hist.member("fN"),
+        " and ",
+        outfile[key].member("fN"),
+    ) from None
+
+
+def hadd_2d(destination, file, key, first):
+    """
+    Args:
+    destination (path-like): Name of the output file or file path.
+    file (ReadOnlyDirectory): ROOT file to read histogram from.
+    key (str): key to reference histogram to be added.
+    first (bool): if True, special case for first of a certain histogram
+        to be added to the new file.
+    """
+    outfile = uproot.open(destination)
+    try:
+        hist = file[key]
+    except ValueError:
+        msg = "Key missing from {file}"
+        raise ValueError(msg) from None
+    if first:
+        member_data = np.array(
+            [
+                hist.member("fEntries"),
+                hist.member("fTsumw"),
+                hist.member("fTsumw2"),
+                hist.member("fTsumwx"),
+                hist.member("fTsumwx2"),
+                hist.member("fTsumwy"),
+                hist.member("fTsumwy2"),
+                hist.member("fTsumwxy"),
+            ]
+        )
+        return uproot.writing.identify.to_TH2x(
+            hist.member("fName"),
+            hist.member("fTitle"),
+            np.ravel(hist.values(flow=True), order="C"),
+            *member_data,
+            np.ravel(hist.variances(flow=True), order="C"),
+            hist.member("fXaxis"),
+            hist.member("fYaxis"),
+        )
+    if hist.member("fN") == outfile[key].member("fN"):
+        member_data = np.array(
+            [
+                hist.member("fEntries"),
+                hist.member("fTsumw"),
+                hist.member("fTsumw2"),
+                hist.member("fTsumwx"),
+                hist.member("fTsumwx2"),
+                hist.member("fTsumwy"),
+                hist.member("fTsumwy2"),
+                hist.member("fTsumwxy"),
+            ]
+        )
+
+        h_sum = uproot.writing.identify.to_TH2x(
+            hist.member("fName"),
+            hist.member("fTitle"),
+            np.ravel(
+                outfile[key].values(flow=True) + hist.values(flow=True), order="C"
+            ),
+            *np.add(
+                np.array(
+                    [
+                        outfile[key].member("fEntries"),
+                        outfile[key].member("fTsumw"),
+                        outfile[key].member("fTsumw2"),
+                        outfile[key].member("fTsumwx"),
+                        outfile[key].member("fTsumwx2"),
+                        outfile[key].member("fTsumwy"),
+                        outfile[key].member("fTsumwy2"),
+                        outfile[key].member("fTsumwxy"),
+                    ]
+                ),
+                member_data,
+            ),
+            np.ravel(
+                outfile[key].variances(flow=True) + hist.variances(flow=True), order="C"
+            ),
+            hist.member("fXaxis"),
+            hist.member("fYaxis"),
+        )
+        outfile.close()
+        return h_sum
+
+    msg = "Bins must be the same for histograms to be added, not "
+    raise ValueError(
+        msg,
+        hist.member("fN"),
+        " and ",
+        outfile[key].member("fN"),
+    ) from None
+
+
+def hadd_3d(destination, file, key, first):
+    """
+    Args:
+    destination (path-like): Name of the output file or file path.
+    file (ReadOnlyDirectory): ROOT file to read histogram from.
+    key (str): key to reference histogram to be added.
+    first (bool): if True, special case for first of a certain histogram
+        to be added to the new file.
+    """
+    outfile = uproot.open(destination)
+    try:
+        hist = file[key]
+    except ValueError:
+        msg = "Key missing from {file}"
+        raise ValueError(msg) from None
+    if first:
+        member_data = np.array(
+            [
+                hist.member("fEntries"),
+                hist.member("fTsumw"),
+                hist.member("fTsumw2"),
+                hist.member("fTsumwx"),
+                hist.member("fTsumwx2"),
+                hist.member("fTsumwy"),
+                hist.member("fTsumwy2"),
+                hist.member("fTsumwxy"),
+                hist.member("fTsumwz"),
+                hist.member("fTsumwz2"),
+                hist.member("fTsumwxz"),
+                hist.member("fTsumwyz"),
+            ]
+        )
+        return uproot.writing.identify.to_TH3x(
+            hist.member("fName"),
+            hist.member("fTitle"),
+            np.ravel(hist.values(flow=True), order="C"),
+            *member_data,
+            np.ravel(hist.variances(flow=True), order="C"),
+            hist.member("fXaxis"),
+            hist.member("fYaxis"),
+            hist.member("fZaxis"),
+        )
+    if hist.member("fN") == outfile[key].member("fN"):
+        member_data = np.add(
+            np.array(
+                [
+                    hist.member("fEntries"),
+                    hist.member("fTsumw"),
+                    hist.member("fTsumw2"),
+                    hist.member("fTsumwx"),
+                    hist.member("fTsumwx2"),
+                    hist.member("fTsumwy"),
+                    hist.member("fTsumwy2"),
+                    hist.member("fTsumwxy"),
+                    hist.member("fTsumwz"),
+                    hist.member("fTsumwz2"),
+                    hist.member("fTsumwxz"),
+                    hist.member("fTsumwyz"),
+                ]
+            ),
+            np.array(
+                [
+                    hist.member("fEntries"),
+                    outfile[key].member("fTsumw"),
+                    outfile[key].member("fTsumw2"),
+                    outfile[key].member("fTsumwx"),
+                    outfile[key].member("fTsumwx2"),
+                    outfile[key].member("fTsumwy"),
+                    outfile[key].member("fTsumwy2"),
+                    outfile[key].member("fTsumwxy"),
+                    outfile[key].member("fTsumwz"),
+                    outfile[key].member("fTsumwz2"),
+                    outfile[key].member("fTsumwxz"),
+                    outfile[key].member("fTsumwyz"),
+                ]
+            ),
+        )
+        h_sum = uproot.writing.identify.to_TH3x(
+            hist.member("fName"),
+            hist.member("fTitle"),
+            np.ravel(
+                outfile[key].values(flow=True) + hist.values(flow=True), order="C"
+            ),
+            *member_data,
+            np.ravel(
+                (outfile[key].variances(flow=True) + hist.variances(flow=True)),
+                order="C",
+            ),
+            hist.member("fXaxis"),
+            hist.member("fYaxis"),
+            hist.member("fZaxis"),
+        )
+        outfile.close()
+        return h_sum
+
+    msg = "Bins must be the same for histograms to be added, not "
+    raise ValueError(
+        msg,
+        hist.member("fN"),
+        " and ",
+        outfile[key].member("fN"),
+    ) from None
+
+
+def hadd(
+    destination,
+    files,
+    *,
+    force=True,
+    append=False,
+    compression="lz4",
+    compression_level=1,
+    skip_bad_files=False,
+    union=True,
+):
+    """
+    Args:
+        destination (path-like): Name of the output file or file path.
+        files (Str or list of str): List of local ROOT files to read histograms from.
+            May contain glob patterns.
+        force (bool): If True, overwrites destination file if it exists. Force and append
+            cannot both be True.
+        append (bool): If True, appends histograms to an existing file. Force and append
+            cannot both be True.
+        compression (str): Sets compression level for root file to write to. Can be one of
+            "ZLIB", "LZMA", "LZ4", or "ZSTD". By default the compression algorithm is "LZ4".
+        compression_level (int): Use a compression level particular to the chosen compressor.
+            By default the compression level is 1.
+        skip_bad_files (bool): If True, skips corrupt or non-existent files without exiting.
+        max_opened_files (int): Limits the number of files to be open at the same time. If 0,
+            this gets set to system limit.
+        union (bool): If True, adds the histograms that have the same name and copies all others
+            to the new file.
+
+    Adds together histograms from local ROOT files of a collection of ROOT files, and writes them to
+        a new or existing ROOT file.
+
+        >>> odapt.add_histograms("destination.root", ["file1_to_hadd.root", "file2_to_hadd.root"])
+
+    """
+    if compression in ("ZLIB", "zlib"):
+        compression_code = uproot.const.kZLIB
+    elif compression in ("LZMA", "lzma"):
+        compression_code = uproot.const.kLZMA
+    elif compression in ("LZ4", "lz4"):
+        compression_code = uproot.const.kLZ4
+    elif compression in ("ZSTD", "zstd"):
+        compression_code = uproot.const.kZSTD
+    else:
+        msg = f"unrecognized compression algorithm: {compression}. Only ZLIB, LZMA, LZ4, and ZSTD are accepted."
+        raise ValueError(msg)
+    p = Path(destination)
+    if Path.is_file(p):
+        if not force and not append:
+            raise FileExistsError
+        if force and append:
+            msg = "Cannot append to a new file. Either force or append can be true."
+            raise ValueError(msg)
+    else:
+        if append:
+            raise FileNotFoundError(
+                "File %s" + destination + " not found. File must exist to append."
+            )
+        file_out = uproot.recreate(
+            destination,
+            compression=uproot.compression.Compression.from_code_pair(
+                compression_code, compression_level
+            ),
+        )
+    if not isinstance(files, list):
+        path = Path(files)
+        files = sorted(path.glob("**/*.root"))
+
+    if len(files) <= 1:
+        msg = "Cannot hadd one file. Use root_to_root to copy a ROOT file."
+        raise ValueError(msg) from None
+
+    with uproot.open(files[0]) as file:
+        keys = file.keys(filter_classname="TH[1|2|3][I|S|F|D|C]", cycle=False)
+    if union:
+        for i, _value in enumerate(files[1:]):
+            with uproot.open(files[i]) as file:
+                keys = np.union1d(
+                    keys,
+                    file.keys(filter_classname="TH[1|2|3][I|S|F|D|C]", cycle=False),
+                )
+                # if files[i + 1] == files[-1]:
+                #     keys_axes = dict(zip(keys, (len(file[j].axes) for j in keys)))
+    else:
+        for i, _value in enumerate(files[1:]):
+            with uproot.open(files[i]) as file:
+                keys = np.intersect1d(
+                    keys,
+                    file.keys(filter_classname="TH[1|2|3][I|S|F|D|C]", cycle=False),
+                )
+                # if files[i + 1] == files[-1]:
+                #     keys_axes = dict(zip(keys, (len(file[j].axes) for j in keys)))
+
+    first = True
+    for input_file in files:
+        p = Path(input_file)
+        if Path.is_file(p):
+            file_out = uproot.update(destination)
+        else:
+            file_out = uproot.recreate(
+                destination,
+                compression=uproot.compression.Compression.from_code_pair(
+                    compression_code, compression_level
+                ),
+            )
+
+        try:
+            file = uproot.open(input_file)
+        except FileNotFoundError:
+            if skip_bad_files:
+                continue
+            msg = "File: {input_file} does not exist or is corrupt."
+            raise FileNotFoundError(msg) from None
+
+        for key in keys:
+            try:
+                file[key]
+            except ValueError:
+                if not union:
+                    continue
+                msg = "Union key filter error."
+                raise ValueError(msg) from None
+            if len(file[key].axes) == 1:
+                h_sum = hadd_1d(destination, file, key, first)
+
+            elif len(file[key].axes) == 2:
+                h_sum = hadd_2d(destination, file, key, first)
+
+            else:
+                h_sum = hadd_3d(destination, file, key, first)
+
+            if h_sum is not None:
+                file_out[key] = h_sum
+
+        first = False
+        file.close()
+
+
+def main():
+    """
+    Implementation of cmd-line executables.
+    """
+    argparser = argparse.ArgumentParser(description="Hadd ROOT histograms with Uproot")
+    argparser.add_argument("destination", type=str, help="path of output file")
+    argparser.add_argument(
+        "input_files",
+        type=str,
+        nargs="+",
+        help="list or directory (glob syntax accepted) of input files",
+    )
+    argparser.add_argument(
+        "-f",
+        "--force",
+        action="store_true",
+        default=True,
+        help="force overwrite of output file",
+    )
+    argparser.add_argument(
+        "-a", "--append", action="store", default=False, help="append to existing file"
+    )
+    argparser.add_argument(
+        "-c",
+        "--compression",
+        action="store",
+        default="lz4",
+        help="set compression level between 1-9",
+    )
+    argparser.add_argument(
+        "-c[0-9]",
+        "--compression_level",
+        action="store",
+        default=1,
+        help="set compression level between 1-9",
+    )
+    argparser.add_argument(
+        "-k",
+        "--skip_bad_files",
+        action="store",
+        default=False,
+        help="corrupt or non-existent input files are ignored",
+    )
+    argparser.add_argument(
+        "-u",
+        action="union",
+        default=True,
+        help="all histograms get copied to new file, only those with same name get added",
+    )
+
+    args = argparser.parse_args()
+
+    hadd(
+        args.destination,
+        args.input_file,
+        force=args.force,
+        append=args.append,
+        compression=args.compression,
+        compression_level=args.compression_level,
+        skip_bad_files=args.skip_bad_files,
+        union=args.union,
+    )
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/samples/__init__.py b/tests/samples/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/samples/file1.root b/tests/samples/file1.root
new file mode 100644
index 0000000..249d632
Binary files /dev/null and b/tests/samples/file1.root differ
diff --git a/tests/samples/file2.root b/tests/samples/file2.root
new file mode 100644
index 0000000..2d48731
Binary files /dev/null and b/tests/samples/file2.root differ
diff --git a/tests/samples/file3.root b/tests/samples/file3.root
new file mode 100644
index 0000000..7458c53
Binary files /dev/null and b/tests/samples/file3.root differ
diff --git a/tests/test_hadd.py b/tests/test_hadd.py
new file mode 100644
index 0000000..87ddf25
--- /dev/null
+++ b/tests/test_hadd.py
@@ -0,0 +1,516 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+import numpy as np
+import pytest
+import uproot
+
+import odapt
+
+ROOT = pytest.importorskip("pyarrow")
+
+
+def write_root_file(hist, path):
+    outHistFile = ROOT.TFile.Open(path, "RECREATE")
+    outHistFile.cd()
+    hist.Write()
+    outHistFile.Close()
+
+
+def generate_1D_gaussian(file_paths):
+    gauss_1 = ROOT.TH1I("name", "title", 5, -4, 4)
+    gauss_1.FillRandom("gaus")
+    gauss_1.Sumw2()
+    gauss_1.SetDirectory(0)
+    outHistFile = ROOT.TFile.Open(file_paths[0], "RECREATE")
+    outHistFile.cd()
+    gauss_1.Write()
+    outHistFile.Close()
+    gauss_1 = uproot.from_pyroot(gauss_1)
+
+    gauss_2 = ROOT.TH1I("name", "title", 5, -4, 4)
+    gauss_2.FillRandom("gaus")
+    gauss_2.Sumw2()
+    gauss_2.SetDirectory(0)
+    outHistFile = ROOT.TFile.Open(file_paths[1], "RECREATE")
+    outHistFile.cd()
+    gauss_2.Write()
+    outHistFile.Close()
+    gauss_2 = uproot.from_pyroot(gauss_2)
+
+    gauss_3 = ROOT.TH1I("name", "title", 5, -4, 4)
+    gauss_3.FillRandom("gaus")
+    gauss_3.Sumw2()
+    gauss_3.SetDirectory(0)
+    outHistFile = ROOT.TFile.Open(file_paths[2], "RECREATE")
+    outHistFile.cd()
+    gauss_3.Write()
+    outHistFile.Close()
+    gauss_3 = uproot.from_pyroot(gauss_3)
+
+    return gauss_1, gauss_2, gauss_3
+
+
+def generate_1D_simple():
+    h1 = ROOT.TH1F("name", "", 10, 0.0, 10.0)
+    data1 = [11.5, 12.0, 9.0, 8.1, 6.4, 6.32, 5.3, 3.0, 2.0, 1.0]
+    for i in range(len(data1)):
+        h1.Fill(i, data1[i])
+
+    outHistFile = ROOT.TFile.Open("tests/file1dim1.root", "RECREATE")
+    outHistFile.cd()
+    h1.Write()
+    outHistFile.Close()
+    h1 = uproot.from_pyroot(h1)
+
+    h2 = ROOT.TH1F("name", "", 10, 0.0, 10.0)
+    data2 = [21.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0]
+
+    for i in range(len(data2)):
+        h2.Fill(i, data2[i])
+
+    outHistFile = ROOT.TFile.Open("tests/file2dim1.root", "RECREATE")
+    outHistFile.cd()
+    h2.Write()
+    outHistFile.Close()
+    h2 = uproot.from_pyroot(h2)
+    return h1, h2
+
+
+def test_simple(tmp_path, file_paths):
+    gauss_1 = ROOT.TH1I("name", "title", 5, -4, 4)
+    gauss_1.FillRandom("gaus")
+    gauss_1.Sumw2()
+    gauss_1.SetDirectory(0)
+    outHistFile = ROOT.TFile.Open(file_paths[0], "RECREATE")
+    outHistFile.cd()
+    gauss_1.Write()
+    outHistFile.Close()
+    h1 = uproot.from_pyroot(gauss_1)
+
+    gauss_2 = ROOT.TH1I("name", "title", 5, -4, 4)
+    gauss_2.FillRandom("gaus")
+    gauss_2.Sumw2()
+    gauss_2.SetDirectory(0)
+    outHistFile = ROOT.TFile.Open(file_paths[1], "RECREATE")
+    outHistFile.cd()
+    gauss_2.Write()
+    outHistFile.Close()
+    h2 = uproot.from_pyroot(gauss_2)
+
+    gauss_3 = ROOT.TH1I("name", "title", 5, -4, 4)
+    gauss_3.FillRandom("gaus")
+    gauss_3.Sumw2()
+    gauss_3.SetDirectory(0)
+    outHistFile = ROOT.TFile.Open(file_paths[2], "RECREATE")
+    outHistFile.cd()
+    gauss_3.Write()
+    outHistFile.Close()
+    h3 = uproot.from_pyroot(gauss_3)
+
+    path = Path(tmp_path)
+    destination = path / "destination.root"
+    odapt.operations.hadd(destination, file_paths, force=True)
+
+    with uproot.open(destination) as file:
+        added = uproot.from_pyroot(
+            gauss_1 + gauss_2 + gauss_3
+        )  # test odapt vs Pyroot histogram adding
+        assert file["name"].member("fN") == added.member("fN")
+        assert file["name"].member("fTsumw") == added.member("fTsumw")
+        assert np.equal(file["name"].values(flow=True), added.values(flow=True)).all
+        assert file["name"].member("fTsumw") == h1.member("fTsumw") + h2.member(
+            "fTsumw"
+        ) + h3.member("fTsumw")
+        assert np.equal(
+            file["name"].values(flow=True),
+            np.array(h1.values(flow=True) + h2.values(flow=True)),
+        ).all
+
+
+test_simple(
+    "tests",
+    [
+        "tests/samples/file1.root",
+        "tests/samples/file2.root",
+        "tests/samples/file3.root",
+    ],
+)
+
+
+def test_3_glob(file_paths):
+    h1, h2, h3 = generate_1D_gaussian(file_paths)
+
+    odapt.operations.hadd("tests/place.root", "tests/samples", force=True)
+
+    with uproot.open("tests/place.root") as file:
+        assert file["name"].member("fN") == h1.member("fN")
+        assert file["name"].member("fTsumw") == h1.member("fTsumw") + h2.member(
+            "fTsumw"
+        ) + h3.member("fTsumw")
+        assert np.equal(
+            file["name"].values(flow=True),
+            np.array(
+                h1.values(flow=True) + h2.values(flow=True) + h3.values(flow=True)
+            ),
+        ).all
+
+
+def simple_1dim_F():
+    h1, h2 = generate_1D_simple()
+    odapt.operations.hadd(
+        "tests/place2.root",
+        ["tests/file1dim1.root", "tests/file2dim1.root"],
+        force=True,
+    )
+
+    with uproot.open("tests/place2.root") as file:
+        assert file["name"].member("fN") == h1.member("fN")
+        assert file["name"].member("fTsumw") == h1.member("fTsumw") + h2.member(
+            "fTsumw"
+        )
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h1.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h2.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h1.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h2.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].values(flow=True),
+            np.array(h1.values(flow=True) + h2.values(flow=True)),
+        ).all
+
+
+def mult_2D_hists():
+    h1 = ROOT.TH2F("name", "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data1 = [
+        [13.5, 11.0, 10.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [11.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [10.5, 10.0, 8.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.6, 1.0],
+        [9.5, 9.0, 8.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.6, 1.0],
+        [8.5, 8.0, 9.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.0, 0.5],
+        [4.5, 7.0, 7.0, 7.2, 6.8, 5.32, 5.3, 2.0, 0.54, 0.25],
+        [3.5, 4.0, 4.0, 4.2, 6.8, 5.32, 5.3, 2.0, 0.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01],
+    ]
+    for i in range(len(data1)):
+        for j in range(len(data1[0])):
+            h1.Fill(i, j, data1[i][j])
+
+    outHistFile = ROOT.TFile.Open("tests/file3dim2.root", "RECREATE")
+    outHistFile.cd()
+    h1.Write()
+    outHistFile.Close()
+    h1 = uproot.from_pyroot(h1)
+
+    h2 = ROOT.TH2F("second", "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data2 = [
+        [21.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [15.5, 13.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.5],
+        [12.5, 10.0, 9.5, 8.2, 6.8, 6.32, 5.2, 3.0, 2.0, 1.25],
+        [9.5, 9.0, 8.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.6, 0.5],
+        [8.5, 8.0, 6.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.0, 0.4],
+        [4.5, 4.0, 4.0, 7.2, 5.8, 5.32, 5.3, 2.0, 0.54, 0.3],
+        [3.5, 4.0, 4.0, 4.2, 5.8, 5.32, 5.3, 2.0, 0.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01, 0.02],
+    ]
+
+    for i in range(len(data2)):
+        for j in range(len(data2[0])):
+            h2.Fill(i, j, data2[i][j])
+
+    outHistFile = ROOT.TFile.Open("tests/file3dim2.root", "UPDATE")
+    outHistFile.cd()
+    h2.Write()
+    outHistFile.Close()
+    h2 = uproot.from_pyroot(h2)
+
+    h3 = ROOT.TH2F("name", "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data3 = [
+        [13.5, 11.0, 10.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [11.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [10.5, 10.0, 8.0, 7.2, 6.8, 5.32, 5.3, 2.5, 1.6, 1.0],
+        [9.5, 9.0, 8.0, 7.2, 6.8, 5.32, 5.3, 3.0, 3.6, 1.0],
+        [8.5, 8.0, 9.0, 7.2, 6.8, 5.32, 5.3, 2.0, 2.0, 0.25],
+        [4.5, 7.0, 7.0, 7.2, 6.8, 5.32, 5.3, 2.0, 0.54, 0.25],
+        [3.5, 4.0, 4.0, 4.2, 6.8, 5.32, 5.3, 2.0, 1.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.51, 0.01, 0.41, 0.01, 0.01, 0.01],
+    ]
+    for i in range(len(data3)):
+        for j in range(len(data3[0])):
+            h3.Fill(i, j, data3[i][j])
+
+    outHistFile = ROOT.TFile.Open("tests/file4dim2.root", "RECREATE")
+    outHistFile.cd()
+    h3.Write()
+    outHistFile.Close()
+    h3 = uproot.from_pyroot(h3)
+
+    h4 = ROOT.TH2F("second", "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data4 = [
+        [21.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [15.5, 13.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.5],
+        [12.5, 10.0, 9.5, 8.2, 6.8, 6.32, 5.2, 3.0, 2.0, 1.25],
+        [9.5, 9.0, 8.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.6, 0.5],
+        [8.5, 8.0, 6.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.0, 0.4],
+        [4.5, 4.0, 4.0, 7.2, 5.8, 5.32, 5.3, 2.0, 0.54, 0.3],
+        [3.5, 4.0, 4.0, 4.2, 5.8, 5.32, 5.3, 2.0, 0.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01, 0.02],
+    ]
+
+    for i in range(len(data4)):
+        for j in range(len(data4[0])):
+            h4.Fill(i, j, data4[i][j])
+
+    outHistFile = ROOT.TFile.Open("tests/file4dim2.root", "UPDATE")
+    outHistFile.cd()
+    h4.Write()
+    outHistFile.Close()
+    h4 = uproot.from_pyroot(h4)
+
+    odapt.operations.hadd(
+        "tests/place2.root",
+        ["tests/file3dim2.root", "tests/file4dim2.root"],
+        force=True,
+    )
+
+    with uproot.open("tests/place2.root") as file:
+        assert file["name"].member("fN") == h1.member("fN")
+        assert file["name"].member("fTsumw") == h1.member("fTsumw") + h3.member(
+            "fTsumw"
+        )
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h1.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h2.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h1.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h2.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h3.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h4.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h3.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h4.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].values(flow=True),
+            np.array(h1.values(flow=True) + h3.values(flow=True)),
+        ).all
+        assert np.equal(
+            file["second"].values(flow=True),
+            np.array(h2.values(flow=True) + h4.values(flow=True)),
+        ).all
+
+
+def simple_2dim_F():
+    fName = "name"
+    h1 = ROOT.TH2F(fName, "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data1 = [
+        [13.5, 11.0, 10.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [11.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [10.5, 10.0, 8.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.6, 1.0],
+        [9.5, 9.0, 8.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.6, 1.0],
+        [8.5, 8.0, 9.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.0, 0.5],
+        [4.5, 7.0, 7.0, 7.2, 6.8, 5.32, 5.3, 2.0, 0.54, 0.25],
+        [3.5, 4.0, 4.0, 4.2, 6.8, 5.32, 5.3, 2.0, 0.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01],
+    ]
+    for i in range(len(data1)):
+        for j in range(len(data1[0])):
+            h1.Fill(i, j, data1[i][j])
+
+    h1 = uproot.from_pyroot(h1)
+
+    h2 = ROOT.TH2F(fName, "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data2 = [
+        [21.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [15.5, 13.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.5],
+        [12.5, 10.0, 9.5, 8.2, 6.8, 6.32, 5.2, 3.0, 2.0, 1.25],
+        [9.5, 9.0, 8.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.6, 0.5],
+        [8.5, 8.0, 6.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.0, 0.4],
+        [4.5, 4.0, 4.0, 7.2, 5.8, 5.32, 5.3, 2.0, 0.54, 0.3],
+        [3.5, 4.0, 4.0, 4.2, 5.8, 5.32, 5.3, 2.0, 0.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01, 0.02],
+    ]
+
+    for i in range(len(data2)):
+        for j in range(len(data2[0])):
+            h2.Fill(i, j, data2[i][j])
+
+    h2 = uproot.from_pyroot(h2)
+
+    odapt.operations.hadd(
+        "tests/place2.root",
+        ["tests/file1dim2.root", "tests/file2dim2.root"],
+        force=True,
+    )
+
+    with uproot.open("tests/place2.root") as file:
+        assert file["name"].member("fN") == h1.member("fN")
+        assert file["name"].member("fTsumw") == h1.member("fTsumw") + h2.member(
+            "fTsumw"
+        )
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h1.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h2.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h1.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h2.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].values(flow=True),
+            np.array(h1.values(flow=True) + h2.values(flow=True)),
+        ).all
+
+
+def simple_2D():
+    h2 = ROOT.TH2F("name", "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data2 = [
+        [21.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [15.5, 13.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.5],
+        [12.5, 10.0, 9.5, 8.2, 6.8, 6.32, 5.2, 3.0, 2.0, 1.25],
+        [9.5, 9.0, 8.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.6, 0.5],
+        [8.5, 8.0, 6.0, 7.2, 5.8, 5.32, 5.3, 2.0, 1.0, 0.4],
+        [4.5, 4.0, 4.0, 7.2, 5.8, 5.32, 5.3, 2.0, 0.54, 0.3],
+        [3.5, 4.0, 4.0, 4.2, 5.8, 5.32, 5.3, 2.0, 0.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01, 0.02],
+    ]
+
+    for i in range(len(data2)):
+        for j in range(len(data2[0])):
+            h2.Fill(i, j, data2[i][j])
+    outHistFile = ROOT.TFile.Open("tests/file2dim2.root", "UPDATE")
+    outHistFile.cd()
+    h2.Write()
+    outHistFile.Close()
+    h2 = uproot.from_pyroot(h2)
+
+    h1 = ROOT.TH2F("name", "", 10, 0.0, 10.0, 8, 0.0, 8.0)
+    data1 = [
+        [13.5, 11.0, 10.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [11.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0],
+        [10.5, 10.0, 8.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.6, 1.0],
+        [9.5, 9.0, 8.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.6, 1.0],
+        [8.5, 8.0, 9.0, 7.2, 6.8, 5.32, 5.3, 2.0, 1.0, 0.5],
+        [4.5, 7.0, 7.0, 7.2, 6.8, 5.32, 5.3, 2.0, 0.54, 0.25],
+        [3.5, 4.0, 4.0, 4.2, 6.8, 5.32, 5.3, 2.0, 0.2, 0.1],
+        [1.5, 1.01, 0.21, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01, 0.01],
+    ]
+    for i in range(len(data1)):
+        for j in range(len(data1[0])):
+            h1.Fill(i, j, data1[i][j])
+
+    outHistFile = ROOT.TFile.Open("tests/file1dim2.root", "RECREATE")
+    outHistFile.cd()
+    h1.Write()
+    outHistFile.Close()
+    h1 = uproot.from_pyroot(h1)
+
+    odapt.operations.hadd(
+        "tests/place2.root",
+        ["tests/file1dim2.root", "tests/file2dim2.root"],
+        force=True,
+    )
+
+    with uproot.open("tests/place2.root") as file:
+        assert file["name"].member("fN") == h1.member("fN")
+        assert file["name"].member("fTsumw") == h1.member("fTsumw") + h2.member(
+            "fTsumw"
+        )
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h1.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fXaxis").edges(flow=True),
+            h2.member("fXaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h1.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].member("fYaxis").edges(flow=True),
+            h2.member("fYaxis").edges(flow=True),
+        ).all
+        assert np.equal(
+            file["name"].values(flow=True),
+            np.array(h1.values(flow=True) + h2.values(flow=True)),
+        ).all
+
+
+mult_2D_hists()
+
+simple_1dim_F()
+
+simple_2dim_F()
+
+
+def break_bins():
+    h1 = ROOT.TH1F("name", "", 8, 0.0, 10.0)
+    data1 = [11.5, 12.0, 9.0, 8.1, 6.4, 6.32, 5.3, 3.0]
+    for i in range(len(data1)):
+        h1.Fill(i, data1[i])
+
+    outHistFile = ROOT.TFile.Open("tests/file1dim1break.root", "RECREATE")
+    outHistFile.cd()
+    h1.Write()
+    outHistFile.Close()
+    h1 = uproot.from_pyroot(h1)
+
+    h2 = ROOT.TH1F("name", "", 10, 0.0, 10.0)
+    data2 = [21.5, 10.0, 9.0, 8.2, 6.8, 6.32, 5.3, 3.0, 2.0, 1.0]
+
+    for i in range(len(data2)):
+        h2.Fill(i, data2[i])
+
+    outHistFile = ROOT.TFile.Open("tests/file2dim1break.root", "RECREATE")
+    outHistFile.cd()
+    h2.Write()
+    outHistFile.Close()
+    h2 = uproot.from_pyroot(h2)
+
+    odapt.operations.hadd(
+        "tests/place2break.root",
+        ["tests/file1dim1break.root", "tests/file2dim1break.root"],
+        force=True,
+    )