Pandas: Create DataFrame from Variables

Table of Contents

1. Core Concepts
2. Typical Usage Methods
3. Common Practices
4. Best Practices
5. Code Examples
6. Conclusion
7. FAQ
8. References

Core Concepts

Variables in Python

In Python, variables are used to store data values. These values can be of different types such as integers, floating - point numbers, strings, lists, dictionaries, etc. When creating a DataFrame from variables, we usually deal with sequences (like lists or tuples) or mappings (like dictionaries).

Pandas DataFrame

A DataFrame in pandas is a 2 - dimensional labeled data structure with columns of potentially different types. It has both row and column labels. Rows are often referred to as index and columns have their own names.

Typical Usage Methods

Using a Dictionary of Lists

The most common way to create a DataFrame from variables is by using a dictionary where the keys represent the column names and the values are lists of equal length representing the data in each column.

import pandas as pd

# Define variables
names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

# Create a dictionary
data = {'Name': names, 'Age': ages}

# Create a DataFrame
df = pd.DataFrame(data)
print(df)

Using Lists of Dictionaries

Another method is to use a list of dictionaries, where each dictionary represents a row in the DataFrame.

import pandas as pd

# Define rows as dictionaries
rows = [
    {'Name': 'Alice', 'Age': 25},
    {'Name': 'Bob', 'Age': 30},
    {'Name': 'Charlie', 'Age': 35}
]

# Create a DataFrame
df = pd.DataFrame(rows)
print(df)

Common Practices

Dealing with Missing Values

When creating a DataFrame from variables, it’s possible to have missing values. You can use None in Python lists to represent missing values, and pandas will convert them to NaN (Not a Number) in the DataFrame.

import pandas as pd

names = ['Alice', 'Bob', None]
ages = [25, None, 35]

data = {'Name': names, 'Age': ages}
df = pd.DataFrame(data)
print(df)

Specifying Column Order

You can specify the order of columns when creating a DataFrame by passing a list of column names as the columns parameter.

import pandas as pd

names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

data = {'Name': names, 'Age': ages}
df = pd.DataFrame(data, columns=['Age', 'Name'])
print(df)

Best Practices

Data Validation

Before creating a DataFrame, make sure that all the lists used to create columns have the same length. Otherwise, pandas will raise a ValueError.

import pandas as pd

names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30]

try:
    data = {'Name': names, 'Age': ages}
    df = pd.DataFrame(data)
except ValueError as e:
    print(f"Error: {e}")

Memory Optimization

If you are dealing with large datasets, consider using appropriate data types for columns. For example, if a column only contains integers in a small range, you can use a smaller integer data type like np.int8 instead of the default np.int64.

import pandas as pd
import numpy as np

names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

data = {'Name': names, 'Age': ages}
df = pd.DataFrame(data)
df['Age'] = df['Age'].astype(np.int8)
print(df.dtypes)

Code Examples

Creating a DataFrame from Multiple Lists

import pandas as pd

# Define variables
countries = ['USA', 'Canada', 'UK']
populations = [331002651, 38005238, 67886011]
capitals = ['Washington, D.C.', 'Ottawa', 'London']

# Create a dictionary
data = {'Country': countries, 'Population': populations, 'Capital': capitals}

# Create a DataFrame
df = pd.DataFrame(data)
print(df)

Creating a DataFrame from a Nested List

import pandas as pd

# Define a nested list
data = [
    ['Alice', 25, 'Engineer'],
    ['Bob', 30, 'Doctor'],
    ['Charlie', 35, 'Teacher']
]

# Define column names
columns = ['Name', 'Age', 'Occupation']

# Create a DataFrame
df = pd.DataFrame(data, columns=columns)
print(df)

Conclusion

Creating a pandas DataFrame from variables is a straightforward and powerful way to structure your data for analysis. By understanding the core concepts, typical usage methods, common practices, and best practices, you can efficiently transform your Python variables into a DataFrame and handle various data scenarios.

FAQ

Q1: What if my lists have different lengths when creating a DataFrame?

A: pandas will raise a ValueError. Make sure all the lists used to create columns have the same length.

Q2: Can I create a DataFrame from variables with different data types?

A: Yes, pandas DataFrame can have columns of different data types. Each column can hold integers, strings, floats, etc.

Q3: How can I add a new column to an existing DataFrame created from variables?

A: You can simply assign a new list or a single value to a new column name. For example:

import pandas as pd

names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]
data = {'Name': names, 'Age': ages}
df = pd.DataFrame(data)
df['Gender'] = ['Female', 'Male', 'Male']
print(df)

References

• Pandas official documentation: https://pandas.pydata.org/docs/
• Python official documentation: https://docs.python.org/3/

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