Python: How to drop a row whose particular column is empty/NaN?

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Question :

Python: How to drop a row whose particular column is empty/NaN?

I have a csv file. I read it:

import pandas as pd
data = pd.read_csv('my_data.csv', sep=',')
data.head()

It has output like:

id    city    department    sms    category
01    khi      revenue      NaN       0
02    lhr      revenue      good      1
03    lhr      revenue      NaN       0

I want to remove all the rows where sms column is empty/NaN. What is efficient way to do it?

Answer #1:

Use dropna with parameter subset for specify column for check NaNs:

data = data.dropna(subset=['sms'])
print (data)
   id city department   sms  category
1   2  lhr    revenue  good         1

Another solution with boolean indexing and notnull:

data = data[data['sms'].notnull()]
print (data)
   id city department   sms  category
1   2  lhr    revenue  good         1

Alternative with query:

print (data.query("sms == sms"))
   id city department   sms  category
1   2  lhr    revenue  good         1

Timings

#[300000 rows x 5 columns]
data = pd.concat([data]*100000).reset_index(drop=True)

In [123]: %timeit (data.dropna(subset=['sms']))
100 loops, best of 3: 19.5 ms per loop

In [124]: %timeit (data[data['sms'].notnull()])
100 loops, best of 3: 13.8 ms per loop

In [125]: %timeit (data.query("sms == sms"))
10 loops, best of 3: 23.6 ms per loop
Answered By: jezrael

Answer #2:

You can use the method dropna for this:

data.dropna(axis=0, subset=('sms', ))

See the documentation for more details on the parameters.

Of course there are multiple ways to do this, and there are some slight performance differences. Unless performance is critical, I would prefer the use of dropna() as it is the most expressive.

import pandas as pd
import numpy as np

i = 10000000

# generate dataframe with a few columns
df = pd.DataFrame(dict(
    a_number=np.random.randint(0,1e6,size=i),
    with_nans=np.random.choice([np.nan, 'good', 'bad', 'ok'], size=i),
    letter=np.random.choice(list('abcdefghijklmnop'), size=i))
                 )

# using notebook %%timeit
a = df.dropna(subset=['with_nans'])
#1.29 s ± 112 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

# using notebook %%timeit
b = df[~df.with_nans.isnull()]
#890 ms ± 59.8 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

# using notebook %%timeit
c = df.query('with_nans == with_nans')
#1.71 s ± 100 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
Answered By: Thijs D

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