Python is a great language for doing data analysis, primarily because of the fantastic ecosystem of data-centric Python packages. Pandas is one of those packages, and makes importing and analyzing data much easier. Sometimes csv file has null values, which are later displayed as NaN in Data Frame. Just like pandas dropna() method manage and remove Null values from a data frame, fillna() manages and let the user replace NaN values with some value of their own.
Syntax:
DataFrame.fillna(value=None, method=None, axis=None, inplace=False, limit=None, downcast=None, **kwargs)
Parameters:
value : Static, dictionary, array, series or dataframe to fill instead of NaN. method : Method is used if user doesn’t pass any value. Pandas has different methods like bfill, backfill or ffill which fills the place with value in the Forward index or Previous/Back respectively. axis: axis takes int or string value for rows/columns. Input can be 0 or 1 for Integer and ‘index’ or ‘columns’ for String inplace: It is a boolean which makes the changes in data frame itself if True. limit : This is an integer value which specifies maximum number of consecutive forward/backward NaN value fills. downcast : It takes a dict which specifies what dtype to downcast to which one. Like Float64 to int64. **kwargs : Any other Keyword arguments
For link to CSV file Used in Code, click here. Example #1: Replacing NaN values with a Static value. Before replacing:
Python3
# importing pandas moduleimport pandas as pd# making data frame from csv filenba = pd.read_csv("nba.csv")nba |
Output: 
After replacing: In the following example, all the null values in College column has been replaced with “No college” string. Firstly, the data frame is imported from CSV and then College column is selected and fillna() method is used on it.
Python
# importing pandas moduleimport pandas as pd# making data frame from csv filenba = pd.read_csv("nba.csv")# replacing na values in college with No collegenba["College"].fillna("No College", inplace = True)nba |
Output: 
Example #2: Using method Parameter In the following example, method is set as ffill and hence the value in the same column replaces the null value. In this case Georgia State replaced null value in college column of row 4 and 5. Similarly, bfill, backfill and pad methods can also be used.
Python
# importing pandas moduleimport pandas as pd# making data frame from csv filenba = pd.read_csv("nba.csv")# replacing na values in college with No collegenba["College"].fillna( method ='ffill', inplace = True)nba |
Output: 
Example #3: Using Limit In this example, a limit of 1 is set in the fillna() method to check if the function stops replacing after one successful replacement of NaN value or not.
Python
# importing pandas moduleimport pandas as pd# making data frame from csv filenba = pd.read_csv("nba.csv")# replacing na values in college with No collegenba["College"].fillna( method ='ffill', limit = 1, inplace = True)nba |
Output: As shown in the output, The college column of 4th row was replaced but 5th one wasn’t since the limit was set 1. 
