Pandas将列转换为不同的dtypes

Question

So I have am using Pandas to create a data frame with some columns which have types bool, int64 and date time. For smaller datasets the dtypes remain but for larger datasets pandas converts all of these to objects. Would anyone know why its doing this and how i can explicitly set type if then?

Reading CSV :

  twitterDataFrame = pandas.read_csv(DataSetLocation)

  twitterDataFrame['CreatedAt'] = twitterDataFrame['CreatedAt'].map(lambda x: pandas.to_datetime(x,dayfirst=True))
  twitterDataFrame['CreatedAtForCalculations'] = twitterDataFrame['CreatedAt']
  twitterDataFrame['InReplyToStatusID'] = twitterDataFrame['InReplyToStatusID'].map(lambda x: True if pandas.notnull(x) else False)
  twitterDataFrame['InReplyToUserID'] = twitterDataFrame['InReplyToUserID'].map(lambda x: True if pandas.notnull(x) else False)
  twitterDataFrame['RetweetCount'] = twitterDataFrame['RetweetCount'].map(lambda x: x if pandas.notnull(x) else 0)
  twitterDataFrame['FavouriteCount'] = twitterDataFrame['FavouriteCount'].map(lambda x: x if pandas.notnull(x) else 0)
  twitterDataFrame['Hashtags'] = twitterDataFrame['Hashtags'].map(lambda x: True if pandas.notnull(x) else False)
  twitterDataFrame['URL'] = twitterDataFrame['URL'].map(lambda x: True if pandas.notnull(x) else False)
  twitterDataFrame['MediaURL'] = twitterDataFrame['MediaURL'].map(lambda x: True if pandas.notnull(x) else False)
  twitterDataFrame['MediaType'] = twitterDataFrame['MediaType'].map(lambda x: x if pandas.notnull(x) else False)
  twitterDataFrame['UserMentionID'] = twitterDataFrame['UserMentionID'].map(lambda x: True if pandas.notnull(x) else False)
  twitterDataFrame['PossiblySensitive'] = twitterDataFrame['PossiblySensitive'].map(lambda x: x if pandas.notnull(x) else 'NoData')

When i print info this is what I get.

None
<class 'pandas.core.frame.DataFrame'>
Int64Index: 21836 entries, 0 to 21835
Data columns (total 17 columns):
CreatedAt                   21836 non-null object
ActualTweet                 21836 non-null object
InReplyToStatusID           21836 non-null bool
InReplyToUserID             21836 non-null bool
UserID                      21836 non-null object
RetweetCount                21836 non-null object
FavouriteCount              21836 non-null object
Hashtags                    21836 non-null bool
URL                         21836 non-null bool
MediaURL                    21836 non-null bool
MediaType                   21836 non-null object
UserMentionID               21836 non-null bool
PossiblySensitive           21836 non-null object
Language                    21836 non-null object
Classifier                  21836 non-null object
TweetLength                 21836 non-null object
CreatedAtForCalculations    21836 non-null object
dtypes: bool(6), object(11)None

For smaller datasets however this works as it should and we get :

None
<class 'pandas.core.frame.DataFrame'>
Int64Index: 8978 entries, 0 to 8977
Data columns (total 17 columns):
CreatedAt                   8978 non-null datetime64[ns]
ActualTweet                 8978 non-null object
InReplyToStatusID           8978 non-null bool
InReplyToUserID             8978 non-null bool
UserID                      8978 non-null int64
RetweetCount                8978 non-null int64
FavouriteCount              8978 non-null int64
Hashtags                    8978 non-null bool
URL                         8978 non-null bool
MediaURL                    8978 non-null bool
MediaType                   8978 non-null object
UserMentionID               8978 non-null bool
PossiblySensitive           8978 non-null object
Language                    8978 non-null object
Trustworthy                 8978 non-null int64
TweetLength                 8978 non-null int64
CreatedAtForCalculations    8978 non-null datetime64[ns]
dtypes: bool(6), datetime64[ns](2), int64(5), object(4)None

Would anyone know why this is and what i can do to fix it?

Answer 1

Here's a nice way to convert an already existing frame's columns from object to something more useful. Normally you don't need to do this as something like read_csv will do conversions for you. But if you have mixed values then these conversions can fail.

See docs here

In [13]: data = """21-01-2014,1
   ....: 31x01x2014,foo
   ....: 01-01-2014,2
   ....: hello,3"""

In [14]: df = pd.DataFrame.from_csv( StringIO(data), index_col=None, header=None )

In [15]: df
Out[15]: 
            0    1
0  21-01-2014    1
1  31x01x2014  foo
2  01-01-2014    2
3       hello    3

In [16]: df.dtypes
Out[16]: 
0    object
1    object
dtype: object

In [17]: df.convert_objects(convert_dates='coerce',convert_numeric=True)
Out[17]: 
           0   1
0 2014-01-21   1
1        NaT NaN
2 2014-01-01   2
3        NaT   3

In [18]: df.convert_objects(convert_dates='coerce',convert_numeric=True).dtypes
Out[18]: 
0    datetime64[ns]
1           float64
dtype: object

This will convert columns that 'look' like datetimes and numbers. Its possible that you want to limit this to certain columns and be a bit more selective. It will only attempt object type columns. Furthermore this is implemented in cython so will be quite fast.

Answer 2

Maybe there is better solution how to find values which can't be converted.

It is my solution using apply()

My data for test:

import pandas as pd
from StringIO import StringIO

data = '''21-01-2014
31x01x2014
01-01-2014
"Hello World"'''

df = pd.DataFrame.from_csv( StringIO(data), index_col=None, header=None )

print df

'''
             0
0   21-01-2014
1   31x01x2014
2   01-01-2014
3  Hello World
'''

I create function which use datetime.datetime.strptime() and try/except to catch (and print) incorrect date.

from datetime import datetime

def test_datetime(x):
    try:
        datetime.strptime(x, "%d-%M-%Y")
    except:
        print 'incorect:', x

then I can use apply() to test all values in column

df[0].apply(test_datetime)

'''
incorect: 31x01x2014
incorect: Hello World
'''

But I can add return True/False in previous function

from datetime import datetime

def test_datetime(x):
    try:
        datetime.strptime(x, "%d-%M-%Y")
        return False
    except:
        return True

to use it this way and get data with index

print df[ df[0].apply(test_datetime) ]

'''
             0
1   31x01x2014
3  Hello World
'''

and run other functions on this rows

df[ df[0].apply(test_datetime) ] = '01-01-2000'

print df

'''
            0
0  21-01-2014
1  01-01-2000
2  01-01-2014
3  01-01-2000
'''