通常，您需要编写一个函数，将您的原始 dict 数据元素转换为 csv 格式的 string 表示形式。
该函数可以编写为DoFn，您可以将其应用于您的数据束PCollection，这会将每个集合元素转换为所需的格式；您可以通过ParDo 将DoFn 应用到您的PCollection 来做到这一点。您还可以将这个DoFn 包装在一个更用户友好的PTransform 中。
你可以在Beam Programming Guide了解更多关于这个过程的信息
这是一个简单的、可翻译的非 Beam 示例：
# Our example list of dictionary elements
test_input = [{'label': 1, 'text': 'Here is a sentence'},
{'label': 2, 'text': 'Another sentence goes here'}]
def convert_my_dict_to_csv_record(input_dict):
""" Turns dictionary values into a comma-separated value formatted string """
return ','.join(map(str, input_dict.values()))
# Our converted list of elements
converted_test_input = [convert_my_dict_to_csv_record(element) for element in test_input]
converted_test_input 将如下所示：
['Here is a sentence,1', 'Another sentence goes here,2']
Beam DictToCSV DoFn 和 PTransform 示例使用DictWriter
from csv import DictWriter
from csv import excel
from cStringIO import StringIO
...
def _dict_to_csv(element, column_order, missing_val='', discard_extras=True, dialect=excel):
""" Additional properties for delimiters, escape chars, etc via an instance of csv.Dialect
Note: This implementation does not support unicode
"""
buf = StringIO()
writer = DictWriter(buf,
fieldnames=column_order,
restval=missing_val,
extrasaction=('ignore' if discard_extras else 'raise'),
dialect=dialect)
writer.writerow(element)
return buf.getvalue().rstrip(dialect.lineterminator)
class _DictToCSVFn(DoFn):
""" Converts a Dictionary to a CSV-formatted String
column_order: A tuple or list specifying the name of fields to be formatted as csv, in order
missing_val: The value to be written when a named field from `column_order` is not found in the input element
discard_extras: (bool) Behavior when additional fields are found in the dictionary input element
dialect: Delimiters, escape-characters, etc can be controlled by providing an instance of csv.Dialect
"""
def __init__(self, column_order, missing_val='', discard_extras=True, dialect=excel):
self._column_order = column_order
self._missing_val = missing_val
self._discard_extras = discard_extras
self._dialect = dialect
def process(self, element, *args, **kwargs):
result = _dict_to_csv(element,
column_order=self._column_order,
missing_val=self._missing_val,
discard_extras=self._discard_extras,
dialect=self._dialect)
return [result,]
class DictToCSV(PTransform):
""" Transforms a PCollection of Dictionaries to a PCollection of CSV-formatted Strings
column_order: A tuple or list specifying the name of fields to be formatted as csv, in order
missing_val: The value to be written when a named field from `column_order` is not found in an input element
discard_extras: (bool) Behavior when additional fields are found in the dictionary input element
dialect: Delimiters, escape-characters, etc can be controlled by providing an instance of csv.Dialect
"""
def __init__(self, column_order, missing_val='', discard_extras=True, dialect=excel):
self._column_order = column_order
self._missing_val = missing_val
self._discard_extras = discard_extras
self._dialect = dialect
def expand(self, pcoll):
return pcoll | ParDo(_DictToCSVFn(column_order=self._column_order,
missing_val=self._missing_val,
discard_extras=self._discard_extras,
dialect=self._dialect)
)
要使用该示例，您可以将test_input 放入PCollection，并将DictToCSV PTransform 应用于PCollection；您可以获取转换后的PCollection 并将其用作WriteToText 的输入。请注意，您必须通过 column_order 参数提供与字典输入元素的键相对应的列名列表或元组；生成的 CSV 格式的字符串列将按照提供的列名的顺序排列。此外，该示例的底层实现不支持unicode。