Memory-efficient operations in Data Analysis Python

Introduction

Memory-efficient operations help you work with large data without using too much computer memory. This keeps your computer fast and avoids crashes.

When working with very large datasets that don't fit in your computer's memory.

When running data analysis on a laptop or device with limited RAM.

When you want to speed up your program by reducing memory use.

When processing data streams or files too big to load all at once.

When you want to save memory to run multiple programs at the same time.

Syntax

Data Analysis Python

import pandas as pd

# Use chunksize to read large CSV in parts
for chunk in pd.read_csv('large_file.csv', chunksize=10000):
    process(chunk)

# Use dtype to reduce memory of columns
df = pd.read_csv('file.csv', dtype={'column1': 'int32', 'column2': 'float32'})

Reading data in chunks lets you handle big files piece by piece.

Setting column data types smaller than default saves memory.

Examples

This reads the CSV file in parts of 5000 rows at a time, so it never loads the whole file at once.

Data Analysis Python

import pandas as pd

# Read CSV in small parts
for chunk in pd.read_csv('data.csv', chunksize=5000):
    print(chunk.shape)

Here, we tell pandas to use smaller data types for columns to save memory.

Data Analysis Python

import pandas as pd

dtypes = {'age': 'int8', 'salary': 'float32'}
df = pd.read_csv('employees.csv', dtype=dtypes)
print(df.memory_usage(deep=True))

Changing text columns to 'category' type saves memory by storing repeated values efficiently.

Data Analysis Python

import numpy as np

# Convert a column to category type
df['city'] = df['city'].astype('category')
print(df.memory_usage(deep=True))

Sample Program

This program creates a big table with 1 million rows. It shows memory used before and after changing data types to save memory.

Data Analysis Python

import pandas as pd
import numpy as np

# Create a large DataFrame with default types
size = 1000000
names = ['Alice', 'Bob', 'Charlie', 'David']

# Default object type for names uses more memory
df = pd.DataFrame({
    'id': np.arange(size),
    'age': np.random.randint(18, 70, size),
    'name': np.random.choice(names, size)
})

print('Memory usage before optimization:')
print(df.memory_usage(deep=True))

# Convert 'name' column to category to save memory
df['name'] = df['name'].astype('category')

# Change 'age' to smaller integer type
df['age'] = df['age'].astype('int8')

print('\nMemory usage after optimization:')
print(df.memory_usage(deep=True))

Important Notes

Using smaller data types can cause errors if values don't fit, so choose carefully.

Converting text columns to 'category' is very effective when many repeated values exist.

Reading data in chunks is useful when you cannot load all data at once.

Summary

Memory-efficient operations help handle big data on limited memory.

Use chunked reading and smaller data types to save memory.

Converting text columns to 'category' type reduces memory for repeated values.

Practice

1. Which of the following is a common method to reduce memory usage when working with large datasets in Python?

easy

A. Increase the size of integer columns to 64-bit

B. Use print statements to debug data

C. Convert text columns to 'category' data type

D. Load the entire dataset into memory at once

Solution

1. Step 1: Understand memory usage of data types

   Text columns with repeated values use more memory as strings than as categories.

2. Step 2: Apply category conversion

   Converting text columns to 'category' type stores unique values once, saving memory.

3. Final Answer:

   Convert text columns to 'category' data type -> Option C

4. Quick Check:

   Category type reduces memory for repeated text [OK]

Hint: Convert repeated text columns to 'category' type to save memory [OK]

Common Mistakes:

• Thinking increasing integer size saves memory
• Loading full dataset without chunking
• Using print for memory optimization

2. Which of the following is the correct syntax to read a large CSV file in chunks of 1000 rows using pandas?

easy

A. pd.read_csv('file.csv', chunksize=1000)

B. pd.read_csv('file.csv', chunk=1000)

C. pd.read_csv('file.csv', size=1000)

D. pd.read_csv('file.csv', batch=1000)

Solution

1. Step 1: Recall pandas chunk reading parameter

   The correct parameter to read in chunks is 'chunksize'.

2. Step 2: Match syntax

   Only pd.read_csv('file.csv', chunksize=1000) uses 'chunksize=1000', which is correct syntax.

3. Final Answer:

   pd.read_csv('file.csv', chunksize=1000) -> Option A

4. Quick Check:

   chunksize=1000 reads file in 1000-row chunks [OK]

Hint: Use 'chunksize' parameter to read CSV in parts [OK]

Common Mistakes:

• Using 'chunk' instead of 'chunksize'
• Using 'size' or 'batch' which are invalid
• Forgetting to specify chunk size

3. What will be the output of the following code snippet?

import pandas as pd
import numpy as np

df = pd.DataFrame({
    'A': np.random.randint(0, 100, 5),
    'B': ['cat', 'dog', 'cat', 'dog', 'cat']
})
df['B'] = df['B'].astype('category')
print(df.memory_usage(deep=True))

medium

A. Raises a TypeError due to invalid astype conversion

B. Prints the DataFrame instead of memory usage

C. Shows memory usage with 'B' column using more memory than object type

D. Shows memory usage with 'B' column using less memory than object type

Solution

1. Step 1: Understand astype('category') effect

   Converting column 'B' to category reduces memory by storing unique values once.

2. Step 2: Interpret memory_usage(deep=True)

   This method shows memory used by each column including actual string data size.

3. Final Answer:

   Shows memory usage with 'B' column using less memory than object type -> Option D

4. Quick Check:

   Category type reduces memory usage [OK]

Hint: astype('category') lowers memory for repeated text columns [OK]

Common Mistakes:

• Expecting error from astype conversion
• Thinking category uses more memory
• Confusing print output with DataFrame display

4. Identify the error in the following code that attempts to reduce memory usage:

import pandas as pd

df = pd.read_csv('data.csv')
df['col'] = df['col'].astype('category')
df = df.astype({'num_col': 'int8'})
print(df.memory_usage())

medium

A. Using 'int8' may cause overflow if values exceed range

B. memory_usage() does not show memory savings

C. astype cannot convert columns after reading CSV

D. No error; code correctly reduces memory usage

Solution

1. Step 1: Check int8 range suitability

   int8 supports values from -128 to 127; larger values cause overflow.

2. Step 2: Identify potential overflow risk

   If 'num_col' has values outside int8 range, conversion causes incorrect data.

3. Final Answer:

   Using 'int8' may cause overflow if values exceed range -> Option A

4. Quick Check:

   int8 range limits can cause data errors [OK]

Hint: Check numeric ranges before downcasting to smaller types [OK]

Common Mistakes:

• Assuming all int conversions are safe
• Believing memory_usage() ignores changes
• Thinking astype fails after CSV read

5. You have a large dataset with a column 'status' containing only three unique string values repeated many times. Which memory-efficient approach is best to reduce memory usage and why? Options: A) Drop the 'status' column to save memory. B) Convert 'status' to string type and keep numeric columns as float64. C) Convert 'status' to category type and downcast numeric columns to smallest possible integer type. D) Convert all columns to object type for flexibility.

hard

A. Drop the 'status' column to save memory.

B. Convert 'status' to category type and downcast numeric columns to smallest possible integer type.

C. Convert 'status' to string type and keep numeric columns as float64.

D. Convert all columns to object type for flexibility.

Solution

1. Step 1: Use category for repeated strings

   Converting 'status' to category stores unique values once, saving memory.

2. Step 2: Downcast numeric columns

   Downcasting numeric columns to smallest integer type reduces memory without losing data.

3. Step 3: Evaluate other options

   Keeping strings as object or dropping columns wastes memory or data; object type uses more memory.

4. Final Answer:

   Convert 'status' to category type and downcast numeric columns to smallest possible integer type. -> Option B

5. Quick Check:

   Category + downcast = best memory saving [OK]

Hint: Use category for repeated strings and downcast numbers [OK]

Common Mistakes:

• Dropping useful columns unnecessarily
• Keeping strings as object wastes memory
• Converting all to object increases memory