I've got following problem. I wrote a 2-dimensional list, where each column is of a different type (bool, str, int, list), to a csv-file. Now I want to read out the data from the csv-file again. But every cell I read is interpreted as a string.
How can I automatically convert the read-in data into the correct type? Or better: Is there a possibility, to tell the csv-reader the correct data-type of each column?
Sample Data (like in csv-file):
IsActive,Type,Price,States
True,Cellphone,34,"[1, 2]"
,FlatTv,3.5,[2]
False,Screen,100.23,"[5, 1]"
True,Notebook, 50,[1]
As the docs explain, the CSV reader doesn't perform automatic data conversion. You have the QUOTE_NONNUMERIC format option, but that would only convert all non-quoted fields into floats. This is a very similar behaviour to other csv readers.
I don't believe Python's csv module would be of any help for this case at all. As others have already pointed out, literal_eval() is a far better choice.
The following does work and converts:
- strings
- int
- floats
- lists
- dictionaries
You may also use it for booleans and NoneType, although these have to be formatted accordingly for literal_eval() to pass. LibreOffice Calc displays booleans in capital letters, when in Python booleans are Capitalized. Also, you would have to replace empty strings with None (without quotes)
I'm writing an importer for mongodb that does all this. The following is part of the code I've written so far.
[NOTE: My csv uses tab as field delimiter. You may want to add some exception handling too]
def getFieldnames(csvFile):
"""
Read the first row and store values in a tuple
"""
with open(csvFile) as csvfile:
firstRow = csvfile.readlines(1)
fieldnames = tuple(firstRow[0].strip('\n').split("\t"))
return fieldnames
def writeCursor(csvFile, fieldnames):
"""
Convert csv rows into an array of dictionaries
All data types are automatically checked and converted
"""
cursor = [] # Placeholder for the dictionaries/documents
with open(csvFile) as csvFile:
for row in islice(csvFile, 1, None):
values = list(row.strip('\n').split("\t"))
for i, value in enumerate(values):
nValue = ast.literal_eval(value)
values[i] = nValue
cursor.append(dict(zip(fieldnames, values)))
return cursor
You have to map your rows:
data = """True,foo,1,2.3,baz
False,bar,7,9.8,qux"""
reader = csv.reader(StringIO.StringIO(data), delimiter=",")
parsed = (({'True':True}.get(row[0], False),
row[1],
int(row[2]),
float(row[3]),
row[4])
for row in reader)
for row in parsed:
print row
results in
(True, 'foo', 1, 2.3, 'baz')
(False, 'bar', 7, 9.8, 'qux')
I know this is a fairly old question, tagged python-2.5, but here's answer that works with Python 3.6+ which might be of interest to folks using more up-to-date versions of the language.
It leverages the built-in typing.NamedTuple class which was added in Python 3.5. What may not be evident from the documentation is that the "type" of each field can be a function.
The example usage code also uses so-called f-string literals which weren't added until Python 3.6, but their use isn't required to do the core data-type transformations.
#!/usr/bin/env python3.6
import ast
import csv
from typing import NamedTuple
class Record(NamedTuple):
""" Define the fields and their types in a record. """
IsActive : bool
Type: str
Price: float
States: ast.literal_eval # Handles string represenation of literals.
@classmethod
def _transform(cls: 'Record', dct: dict) -> dict:
""" Convert string values in given dictionary to corresponding Record
field type.
"""
return {field: cls._field_types[field](value)
for field, value in dct.items()}
filename = 'test_transform.csv'
with open(filename, newline='') as file:
for i, row in enumerate(csv.DictReader(file)):
row = Record._transform(row)
print(f'row {i}: {row}')
Output:
row 0: {'IsActive': True, 'Type': 'Cellphone', 'Price': 34.0, 'States': [1, 2]}
row 1: {'IsActive': False, 'Type': 'FlatTv', 'Price': 3.5, 'States': [2]}
row 2: {'IsActive': True, 'Type': 'Screen', 'Price': 100.23, 'States': [5, 1]}
row 3: {'IsActive': True, 'Type': 'Notebook', 'Price': 50.0, 'States': [1]}
Generalizing this by creating a base class with just the generic classmethod in it is not simple because of the way typing.NamedTuple is implemented.
To avoid that issue, in Python 3.7+, a dataclasses.dataclass could be used instead because they do not have the inheritance issue — so creating a generic base class that can be reused is simple:
#!/usr/bin/env python3.7
import ast
import csv
from dataclasses import dataclass, fields
from typing import Type, TypeVar
T = TypeVar('T', bound='GenericRecord')
class GenericRecord:
""" Generic base class for transforming dataclasses. """
@classmethod
def _transform(cls: Type[T], dict_: dict) -> dict:
""" Convert string values in given dictionary to corresponding type. """
return {field.name: field.type(dict_[field.name])
for field in fields(cls)}
@dataclass
class CSV_Record(GenericRecord):
""" Define the fields and their types in a record.
Field names must match column names in CSV file header.
"""
IsActive : bool
Type: str
Price: float
States: ast.literal_eval # Handles string represenation of literals.
filename = 'test_transform.csv'
with open(filename, newline='') as file:
for i, row in enumerate(csv.DictReader(file)):
row = CSV_Record._transform(row)
print(f'row {i}: {row}')
In one sense it's not really very important which one you use because an instance of the class in never created — using one is just a clean way of specifying and holding a definition of the field names and their type in a record data-structure.
Props to Jon Clements and cortopy for teaching me about ast.literal_eval! Here's what I ended up going with (Python 2; changes for 3 should be trivial):
from ast import literal_eval
from csv import DictReader
import csv
def csv_data(filepath, **col_conversions):
"""Yield rows from the CSV file as dicts, with column headers as the keys.
Values in the CSV rows are converted to Python values when possible,
and are kept as strings otherwise.
Specific conversion functions for columns may be specified via
`col_conversions`: if a column's header is a key in this dict, its
value will be applied as a function to the CSV data. Specify
`ColumnHeader=str` if all values in the column should be interpreted
as unquoted strings, but might be valid Python literals (`True`,
`None`, `1`, etc.).
Example usage:
>>> csv_data(filepath,
... VariousWordsIncludingTrueAndFalse=str,
... NumbersOfVaryingPrecision=float,
... FloatsThatShouldBeRounded=round,
... **{'Column Header With Spaces': arbitrary_function})
"""
def parse_value(key, value):
if key in col_conversions:
return col_conversions[key](value)
try:
# Interpret the string as a Python literal
return literal_eval(value)
except Exception:
# If that doesn't work, assume it's an unquoted string
return value
with open(filepath) as f:
# QUOTE_NONE: don't process quote characters, to avoid the value
# `"2"` becoming the int `2`, rather than the string `'2'`.
for row in DictReader(f, quoting=csv.QUOTE_NONE):
yield {k: parse_value(k, v) for k, v in row.iteritems()}
(I'm a little wary that I might have missed some corner cases involving quoting. Please comment if you see any issues!)
An alternative (although it seems a bit extreme) in lieu of using ast.literal_eval is the pyparsing module available on PyPi - and see if the http://pyparsing.wikispaces.com/file/view/parsePythonValue.py code sample is either appropriate for what you require, or can be easily adapted.
I love @martineau's answer. It's very clean.
One thing I needed was to convert only a couple of values and leave all the other fields as strings, like having strings as default and just updating the type for specific keys.
To do that, just replace this line:
row = CSV_Record._transform(row)
by this one:
row.update(CSV_Record._transform(row))
The 'update' function updates the variable row directly, merging the raw data from the csv extract with the values converted to the correct type by the '_transform' method.
Note there is no 'row = ' in the updated version.
Hope this will help in case anyone has a similar requirement.
(PS: I'm quite new to posting on stackoverflow, so please let me know if the above is not clear)
来源：https://stackoverflow.com/questions/11665628/read-data-from-csv-file-and-transform-to-correct-data-type