fillna Keywords¶

The fillna() method accepts several keyword arguments that control how missing values are filled.

method Keyword¶

Propagate non-null values forward or backward.

1. Forward Fill (ffill)¶

import pandas as pd

url = "https://raw.githubusercontent.com/codebasics/py/master/pandas/5_handling_missing_data_fillna_dropna_interpolate/weather_data.csv"
df = pd.read_csv(url, index_col='day', parse_dates=True)
print(df)

dg = df.fillna(method='ffill')
print(dg)

Forward fill propagates the last valid observation forward.

2. Backward Fill (bfill)¶

dg = df.fillna(method='bfill')
print(dg)

Backward fill uses the next valid observation to fill gaps.

3. pad Alias¶

df.fillna(method='pad')    # Same as ffill
df.fillna(method='backfill')  # Same as bfill

axis Keyword¶

Specify the axis along which to fill missing values.

1. Fill Along Rows (axis=0)¶

dg = df.fillna(method='ffill', axis=0)
print(dg)

Default behavior: fill down columns.

2. Fill Along Columns (axis=1)¶

dg = df.fillna(method='ffill', axis=1)
print(dg)

Fill across rows from left to right.

3. Numeric vs String Columns¶

When filling along axis=1, be aware that mixed types may cause issues.

limit Keyword¶

Limit the number of consecutive NaN values to fill.

1. Limit Forward Fill¶

dg = df.fillna(method='ffill', limit=1)
print(dg)

Only fills up to 1 consecutive NaN value.

2. Preventing Over-filling¶

# If there are 3 consecutive NaNs and limit=2
# Only the first 2 will be filled

3. Use Case¶

# In time series, limit prevents filling across
# long gaps where forward fill may be inappropriate
df['price'].fillna(method='ffill', limit=5)

Combined Keywords¶

Use multiple keywords together for precise control.

1. Forward Fill with Limit¶

df.fillna(method='ffill', axis=0, limit=2)

2. Backward Fill with Limit¶

df.fillna(method='bfill', axis=0, limit=1)

3. Fill Strategy¶

# First forward fill, then backward fill remaining
df_filled = df.fillna(method='ffill').fillna(method='bfill')

Modern Syntax¶

In recent pandas versions, prefer explicit methods over the method keyword.

1. ffill Method¶

df.ffill()           # Forward fill
df.ffill(limit=2)    # With limit

2. bfill Method¶

df.bfill()           # Backward fill
df.bfill(limit=1)    # With limit

3. Deprecation Note¶

The method parameter in fillna is deprecated in newer pandas versions:

# Deprecated
df.fillna(method='ffill')

# Preferred
df.ffill()