pandas DataFrame.astype() – Examples

DataFrame.astype() function is used to cast a column data type (dtype) in pandas object, it supports String, flat, date, int, datetime any many other dtypes supported by Numpy. This comes in handy when you wanted to cast the DataFrame column from one data type to another.

pandas astype() Key Points –

1. It is used to cast datatype (dtype).
2. Supports changing multiple data types using Dict.
3. Supports all data types that comes with Numpy.

1. DataFrame.astype() Syntax

Following is a syntax of the DataFrame.astype(). This function takes dtype, copy, and errors params.

# astype() Syntax
DataFrame.astype(dtype, copy=True, errors='raise')

Following are the parameters of astype() function.

• dtype – Accepts a numpy.dtype or Python type to cast entire pandas object to the same type. Use {col: dtype, …}, where col is a column label and dtype is a numpy.dtype or Python type to cast one or more of the DataFrame’s columns.
• copy -Default True. Return a copy when copy=True (be very careful setting copy=False as changes to values then may propagate to other pandas objects).
• errors – Default raise.
  • Use ‘raise’ to generate exception when unable to cast due to invalid data for type.
  • Use ‘ignore’ to not raise exception (supress errors/exceptions). On error return original object.

2. DataFrame.astype() – Cast All Columns Data Type (dtype)

By default pandas astype() function tries to cast all DataFrame columns to specified numpy.dtype or Python types (int, string, float, date, datetime) . If any of the columns are unable to cast due to the invalid data or nan, it raises the error ‘ValueError: invalid literal’ and fails the operation.

The below example demonstrates casting all columns data types.

# DataFrame.astype() - Cast All Columns Data Type (dtype)
import pandas as pd
import numpy as np
# Create DataFrame from Dictionary
technologies = {
    'Fee' :["20000","25000","26000"],
    'Discount':["1000","2300","1500"]
              }
df = pd.DataFrame(technologies)
print(df.dtypes)

# Output:
# Fee         object
# Discount    object
# dtype: object

DataFrame.dtypes returns the Column name and dtypes for all DataFrame columns. Note that the above DataFrame has object types for all columns.

Now let’s cast the data type to 64-bit signed integer, you can use numpy.int64,numpy.int_, int64 or int as param. To cast to 32-bit signed integer, use numpy.int32, int32.

# Cast all columns to int
df = df.astype(np.int64)
df = df.astype('int64')
df = df.astype('int')

print(df.dtypes) 

# output:
# Fee         int64
# Discount    int64
# dtype: object

Notice that it updated all columns with the new dtype.

Let’s cast it to String, using numpy.str_ or string.

# Cast all columns to string
df = df.astype('string')
print(df.dtypes)

# Output:
# Fee         string
# Discount    string
# dtype: object

Let’s cast it to float type using numpy.float64, numpy.float_, float

# Cast all columns to float
df = df.astype('float')
print(df.dtypes)

# Output:
# Fee         float
# Discount    flat
# dtype: object

3. Change Specific Column Type

You can also change the specific column type by using Series.astype() function, since each column on DataFrame is pandas Series, I will get the column from DataFrame as Series and use astype() function. In the below example df.Fee or df['Fee'] returns Series object.

# Cast specific column type
df.Fee = df.Fee.astype('int')
(or)
df.Fee = df['Fee'].astype('int')
print(df.dtypes)

# Output:
# Fee          int64
# Discount    object
# dtype: object

4. astype() – Cast Multiple Columns Using Dict

dtype param of the astype() function also supports Dictionary in format {col: dtype, …} where col is a column label and dtype is a numpy.dtype or Python type (int, string, float, date, datetime) to cast one or multiple DataFrame columns.

# Astype() - Cast Multiple Columns Using Dict 
import pandas as pd
import numpy as np
# Create DataFrame from Dictionary
technologies = {
    'Courses':["Spark","PySpark","Hadoop"],
    'Fee' :["20000","25000","26000"],
    'Duration':['30day','40days','35days'],
    'Discount':["1000","2300","1500"]
              }

df = pd.DataFrame(technologies)
print(df.dtypes)

# Output:
# Courses     object
# Fee         object
# Duration    object
# Discount    object
# dtype: object

Now, by using the pandas DataFrame.astype() function, cast the Courses column to string, Fee column to int and Discount column to float.

# Apply cast type for multiple columns
df2 = df.astype({'Courses':'string','Fee':'int','Discount':'float'})
print(df2.dtypes)

# Output:
# Courses      string
# Fee           int64
# Duration     object
# Discount    float64
# dtype: object

5. astype() with raise or ignore Error

Finally, let’s see how you can raise or ignore the error while casting, to do so you should use errors param. By default, it uses raise as a value meaning generate an exception when unable to cast due to invalid data for type.

From our DataFrame Courses column have string data, let’s cast this to int and see what happens.

# Raise error when unable to cast
df.Courses = df.Courses.astype('int')

# Output:
# ValueError: invalid literal for int() with base 10: 'Spark'

As you see, it raised the error when unable to cast. Now let’s suppress the exception using ignore value on errors param. With this, when errors happen it ignores the error and returns the same object without updating.

# Ignore error when unable to cast
df.Courses = df.Courses.astype('int', errors='ignore')
print(df.dtypes)

# Output:
# Courses      string
# Fee           int64
# Duration     object
# Discount    float64
# dtype: object

Conclusion

In this article, I have explained the pandas DataFrame.astype() syntax, examples of casting entire DataFrame, specific column, multiple columns to numpy.dtype or Python type (int, string, float, date, datetime).

Related Articles

• Different Ways to Change Data Type in pandas
• Pandas Convert Column to Int in DataFrame
• Sort Pandas DataFrame by Date (Datetime)
• Convert Pandas Series to String
• Pandas Convert String to Integer
• Pandas Convert Column to Int in DataFrame
• Pandas Replace Values based on Condition

References

• https://numpy.org/doc/stable/reference/generated/numpy.ndarray.astype.html#numpy.ndarray.astype