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#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
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# ==============================================================================
"""Dirichlet partitioner class that works with Hugging Face Datasets."""
import warnings
from typing import Optional, Union
import numpy as np
import datasets
from flwr_datasets.common.typing import NDArrayFloat
from flwr_datasets.partitioner.partitioner import Partitioner
# pylint: disable=R0902, R0912, R0914
[docs]class DirichletPartitioner(Partitioner):
"""Partitioner based on Dirichlet distribution.
Implementation based on Bayesian Nonparametric Federated Learning of Neural Networks
https://arxiv.org/abs/1905.12022.
The algorithm sequentially divides the data with each label. The fractions of the
data with each label is drawn from Dirichlet distribution and adjusted in case of
balancing. The data is assigned. In case the `min_partition_size` is not satisfied
the algorithm is run again (the fractions will change since it is a random process
even though the alpha stays the same).
The notion of balancing is explicitly introduced here (not mentioned in paper but
implemented in the code). It is a mechanism that excludes the partition from
assigning new samples to it if the current number of samples on that partition
exceeds the average number that the partition would get in case of even data
distribution. It is controlled by`self_balancing` parameter.
Parameters
----------
num_partitions : int
The total number of partitions that the data will be divided into.
partition_by : str
Column name of the labels (targets) based on which Dirichlet sampling works.
alpha : Union[int, float, List[float], NDArrayFloat]
Concentration parameter to the Dirichlet distribution
min_partition_size : int
The minimum number of samples that each partitions will have (the sampling
process is repeated if any partition is too small).
self_balancing : bool
Whether assign further samples to a partition after the number of samples
exceeded the average number of samples per partition. (True in the original
paper's code although not mentioned in paper itself).
shuffle: bool
Whether to randomize the order of samples. Shuffling applied after the
samples assignment to partitions.
seed: int
Seed used for dataset shuffling. It has no effect if `shuffle` is False.
Examples
--------
>>> from flwr_datasets import FederatedDataset
>>> from flwr_datasets.partitioner import DirichletPartitioner
>>>
>>> partitioner = DirichletPartitioner(num_partitions=10, partition_by="label",
>>> alpha=0.5, min_partition_size=10,
>>> self_balancing=True)
>>> fds = FederatedDataset(dataset="mnist", partitioners={"train": partitioner})
>>> partition = fds.load_partition(0)
>>> print(partition[0]) # Print the first example
{'image': <PIL.PngImagePlugin.PngImageFile image mode=L size=28x28 at 0x127B92170>,
'label': 4}
>>> partition_sizes = [
>>> len(fds.load_partition(partition_id)) for partition_id in range(10)
>>> ]
>>> print(sorted(partition_sizes))
[2134, 2615, 3646, 6011, 6170, 6386, 6715, 7653, 8435, 10235]
"""
def __init__( # pylint: disable=R0913
self,
num_partitions: int,
partition_by: str,
alpha: Union[int, float, list[float], NDArrayFloat],
min_partition_size: int = 10,
self_balancing: bool = False,
shuffle: bool = True,
seed: Optional[int] = 42,
) -> None:
super().__init__()
# Attributes based on the constructor
self._num_partitions = num_partitions
self._check_num_partitions_greater_than_zero()
self._alpha: NDArrayFloat = self._initialize_alpha(alpha)
self._partition_by = partition_by
self._min_partition_size: int = min_partition_size
self._self_balancing = self_balancing
self._shuffle = shuffle
self._seed = seed
self._rng = np.random.default_rng(seed=self._seed) # NumPy random generator
# Utility attributes
# The attributes below are determined during the first call to load_partition
self._avg_num_of_samples_per_partition: Optional[float] = None
self._unique_classes: Optional[Union[list[int], list[str]]] = None
self._partition_id_to_indices: dict[int, list[int]] = {}
self._partition_id_to_indices_determined = False
[docs] def load_partition(self, partition_id: int) -> datasets.Dataset:
"""Load a partition based on the partition index.
Parameters
----------
partition_id : int
the index that corresponds to the requested partition
Returns
-------
dataset_partition : Dataset
single partition of a dataset
"""
# The partitioning is done lazily - only when the first partition is
# requested. Only the first call creates the indices assignments for all the
# partition indices.
self._check_num_partitions_correctness_if_needed()
self._determine_partition_id_to_indices_if_needed()
return self.dataset.select(self._partition_id_to_indices[partition_id])
@property
def num_partitions(self) -> int:
"""Total number of partitions."""
self._check_num_partitions_correctness_if_needed()
self._determine_partition_id_to_indices_if_needed()
return self._num_partitions
def _initialize_alpha(
self, alpha: Union[int, float, list[float], NDArrayFloat]
) -> NDArrayFloat:
"""Convert alpha to the used format in the code a NDArrayFloat.
The alpha can be provided in constructor can be in different format for user
convenience. The format into which it's transformed here is used throughout the
code for computation.
Parameters
----------
alpha : Union[int, float, List[float], NDArrayFloat]
Concentration parameter to the Dirichlet distribution
Returns
-------
alpha : NDArrayFloat
Concentration parameter in a format ready to used in computation.
"""
if isinstance(alpha, int):
alpha = np.array([float(alpha)], dtype=float).repeat(self._num_partitions)
elif isinstance(alpha, float):
alpha = np.array([alpha], dtype=float).repeat(self._num_partitions)
elif isinstance(alpha, list):
if len(alpha) != self._num_partitions:
raise ValueError(
"If passing alpha as a List, it needs to be of length of equal to "
"num_partitions."
)
alpha = np.asarray(alpha)
elif isinstance(alpha, np.ndarray):
# pylint: disable=R1720
if alpha.ndim == 1 and alpha.shape[0] != self._num_partitions:
raise ValueError(
"If passing alpha as an NDArray, its length needs to be of length "
"equal to num_partitions."
)
elif alpha.ndim == 2:
alpha = alpha.flatten()
if alpha.shape[0] != self._num_partitions:
raise ValueError(
"If passing alpha as an NDArray, its size needs to be of length"
" equal to num_partitions."
)
else:
raise ValueError("The given alpha format is not supported.")
if not (alpha > 0).all():
raise ValueError(
f"Alpha values should be strictly greater than zero. "
f"Instead it'd be converted to {alpha}"
)
return alpha
def _determine_partition_id_to_indices_if_needed(
self,
) -> None:
"""Create an assignment of indices to the partition indices."""
if self._partition_id_to_indices_determined:
return
# Generate information needed for Dirichlet partitioning
self._unique_classes = self.dataset.unique(self._partition_by)
assert self._unique_classes is not None
# This is needed only if self._self_balancing is True (the default option)
self._avg_num_of_samples_per_partition = (
self.dataset.num_rows / self._num_partitions
)
# Change targets list data type to numpy
targets = np.array(self.dataset[self._partition_by])
# Repeat the sampling procedure based on the Dirichlet distribution until the
# min_partition_size is reached.
sampling_try = 0
while True:
# Prepare data structure to store indices assigned to partition ids
partition_id_to_indices: dict[int, list[int]] = {}
for nid in range(self._num_partitions):
partition_id_to_indices[nid] = []
# Iterated over all unique labels (they are not necessarily of type int)
for k in self._unique_classes:
# Access all the indices associated with class k
indices_representing_class_k = np.nonzero(targets == k)[0]
# Determine division (the fractions) of the data representing class k
# among the partitions
class_k_division_proportions = self._rng.dirichlet(self._alpha)
nid_to_proportion_of_k_samples = {}
for nid in range(self._num_partitions):
nid_to_proportion_of_k_samples[nid] = class_k_division_proportions[
nid
]
# Balancing (not mentioned in the paper but implemented)
# Do not assign additional samples to the partition if it already has
# more than the average numbers of samples per partition. Note that it
# might especially affect classes that are later in the order. This is
# the reason for more sparse division that the alpha might suggest.
if self._self_balancing:
assert self._avg_num_of_samples_per_partition is not None
for nid in nid_to_proportion_of_k_samples.copy():
if (
len(partition_id_to_indices[nid])
> self._avg_num_of_samples_per_partition
):
nid_to_proportion_of_k_samples[nid] = 0
# Normalize the proportions such that they sum up to 1
sum_proportions = sum(nid_to_proportion_of_k_samples.values())
for nid, prop in nid_to_proportion_of_k_samples.copy().items():
nid_to_proportion_of_k_samples[nid] = prop / sum_proportions
# Determine the split indices
cumsum_division_fractions = np.cumsum(
list(nid_to_proportion_of_k_samples.values())
)
cumsum_division_numbers = cumsum_division_fractions * len(
indices_representing_class_k
)
# [:-1] is because the np.split requires the division indices but the
# last element represents the sum = total number of samples
indices_on_which_split = cumsum_division_numbers.astype(int)[:-1]
split_indices = np.split(
indices_representing_class_k, indices_on_which_split
)
# Append new indices (coming from class k) to the existing indices
for nid, indices in partition_id_to_indices.items():
indices.extend(split_indices[nid].tolist())
# Determine if the indices assignment meets the min_partition_size
# If it does not mean the requirement repeat the Dirichlet sampling process
# Otherwise break the while loop
min_sample_size_on_client = min(
len(indices) for indices in partition_id_to_indices.values()
)
if min_sample_size_on_client >= self._min_partition_size:
break
sample_sizes = [
len(indices) for indices in partition_id_to_indices.values()
]
alpha_not_met = [
self._alpha[i]
for i, ss in enumerate(sample_sizes)
if ss == min(sample_sizes)
]
mssg_list_alphas = (
(
"Generating partitions by sampling from a list of very wide range "
"of alpha values can be hard to achieve. Try reducing the range "
f"between maximum ({max(self._alpha)}) and minimum alpha "
f"({min(self._alpha)}) values or increasing all the values."
)
if len(self._alpha.flatten().tolist()) > 0
else ""
)
warnings.warn(
f"The specified min_partition_size ({self._min_partition_size}) was "
f"not satisfied for alpha ({alpha_not_met}) after "
f"{sampling_try} attempts at sampling from the Dirichlet "
f"distribution. The probability sampling from the Dirichlet "
f"distribution will be repeated. Note: This is not a desired "
f"behavior. It is recommended to adjust the alpha or "
f"min_partition_size instead. {mssg_list_alphas}",
stacklevel=1,
)
if sampling_try == 10:
raise ValueError(
"The max number of attempts (10) was reached. "
"Please update the values of alpha and try again."
)
sampling_try += 1
# Shuffle the indices not to have the datasets with targets in sequences like
# [00000, 11111, ...]) if the shuffle is True
if self._shuffle:
for indices in partition_id_to_indices.values():
# In place shuffling
self._rng.shuffle(indices)
self._partition_id_to_indices = partition_id_to_indices
self._partition_id_to_indices_determined = True
def _check_num_partitions_correctness_if_needed(self) -> None:
"""Test num_partitions when the dataset is given (in load_partition)."""
if not self._partition_id_to_indices_determined:
if self._num_partitions > self.dataset.num_rows:
raise ValueError(
"The number of partitions needs to be smaller than the number of "
"samples in the dataset."
)
def _check_num_partitions_greater_than_zero(self) -> None:
"""Test num_partition left sides correctness."""
if not self._num_partitions > 0:
raise ValueError("The number of partitions needs to be greater than zero.")