How to Read a File into an Array in Python

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I once built a small leave-tracking script for a 10-person U.S. operations team. HR exported employee requests from Excel as a CSV file, but the script needed that data in memory before it could validate dates, count leave days, and create a weekly report. The first job was simple: read the file into an array in Python.

The word array causes some confusion in Python. A standard Python list usually works as the array for text and mixed data. Numeric projects may need a real NumPy array instead. This guide shows the practical choices, complete code, expected output, and the reasons behind each approach.

What Does Reading a File into an Array Mean?

Reading a file into an array means loading file content from disk into a Python data structure. Once the data sits in memory, your program can loop through it, filter it, sort it, validate it, or calculate results.

For most beginner projects, the target structure is a list. A list stores items in order and lets you access each item by its index. Understanding the differences between lists and dictionaries in Python helps when choosing the final structure.

Suppose leave_reasons.txt contains:

Vacation
Medical appointment
Jury duty
Personal day

Python can read those lines into this list:

[
    "Vacation",
    "Medical appointment",
    "Jury duty",
    "Personal day"
]

That structure works well because each file line represents one business value. Your code can now check whether a submitted reason matches an approved leave category.

Python 3.10 or later works for these examples. NumPy requires a separate installation.

Set Up the Example Files

Create a folder named leave_file_project on your local machine. Keep the Python script and sample files together while learning:

leave_file_project/
├── app.py
├── leave_reasons.txt
├── leave_requests.csv
├── leave_requests.json
└── hours.txt

This one-folder structure removes path confusion and keeps the beginner project easy to run.

Add the following content to leave_requests.csv:

employee,start_date,end_date,leave_type,status
Mia Johnson,2026-07-20,2026-07-22,Vacation,Pending
Ethan Davis,2026-07-24,2026-07-24,Jury Duty,Approved
Olivia Brown,2026-08-03,2026-08-04,Personal,Pending

The YYYY-MM-DD dates sort as text and work with Python’s datetime module.

How to Read a File into an Array in Python

The most direct approach uses the built-in open() function and a with statement. Learning how to open a file in Python correctly matters because the with statement closes it automatically, even when an error interrupts your code.

Add this code to app.py:

with open("leave_reasons.txt", "r", encoding="utf-8") as file:
    leave_reasons = file.readlines()

print(leave_reasons)

Output:

['Vacation\n', 'Medical appointment\n', 'Jury duty\n', 'Personal day\n']

I executed the above example code and added the screenshot below.

Read a File into an Array in Python

The "r" argument opens the file in read mode. The encoding="utf-8" argument tells Python how to convert stored bytes into readable text. UTF-8 handles common U.S. English text and names with accented characters.

The result contains \n, which represents a newline character. Newlines often cause failed comparisons because "Vacation\n" does not equal "Vacation".

Use a list comprehension to remove surrounding whitespace:

with open("leave_reasons.txt", "r", encoding="utf-8") as file:
    leave_reasons = [line.strip() for line in file]

print(leave_reasons)
print(leave_reasons[0])

Output:

['Vacation', 'Medical appointment', 'Jury duty', 'Personal day']
Vacation

I executed the above example code and added the screenshot below.

How to Read a File into an Array in Python

A list comprehension builds a list while looping through another object. Here, the Python strip function removes spaces, tabs, and newline characters from each line.

This pattern safely closes the file and cleans values immediately.

Skip Empty Lines While Reading the File

Real files often contain blank lines. If blank entries have no business meaning, filter them during the same list comprehension:

with open("leave_reasons.txt", "r", encoding="utf-8") as file:
    leave_reasons = [
        line.strip()
        for line in file
        if line.strip()
    ]

print(leave_reasons)

Output:

['Vacation', 'Medical appointment', 'Jury duty', 'Personal day']

The condition keeps nonempty strings because Python treats an empty string as false. It prevents blank leave types from entering reports.

Pro Tip: In my experience, strip() fixes more file-import bugs than developers expect. I clean each line at the boundary, before any validation or comparison runs.

Read the Entire File and Split It into an Array

You can also call read() to load the complete file as one string. Then use splitlines() to create the list:

with open("leave_reasons.txt", "r", encoding="utf-8") as file:
    leave_reasons = file.read().splitlines()

print(leave_reasons)

Output:

['Vacation', 'Medical appointment', 'Jury duty', 'Personal day']

I executed the above example code and added the screenshot below.

Read a File into an Python Array

The splitlines() technique removes common Windows, macOS, and Linux line endings. It produces cleaner results than readlines() when newlines have no value.

However, read() loads everything into memory. It suits configuration files and small exports but may exhaust memory with multi-gigabyte files.

For a large file, read the Python file line by line instead:

approved_count = 0

with open("large_leave_export.txt", "r", encoding="utf-8") as file:
    for line in file:
        if "Approved" in line:
            approved_count += 1

print(f"Approved requests: {approved_count}")

Output:

Approved requests: 1842

This version intentionally avoids a full array. Streaming saves memory when you only need a count or filtered result.

Read a CSV File into an Array in Python

A CSV file stores comma-separated values. Do not use line.split(",") because valid fields may contain quoted commas, such as "Austin, Texas". Python’s csv module understands these rules, and a streaming approach also helps you read large CSV files in Python.

Create an Array of Rows

Use csv.reader() when you want a list containing one list per row:

import csv

with open("leave_requests.csv", "r", newline="", encoding="utf-8") as file:
    reader = csv.reader(file)
    leave_requests = list(reader)

print(leave_requests[0])
print(leave_requests[1])

Output:

['employee', 'start_date', 'end_date', 'leave_type', 'status']
['Mia Johnson', '2026-07-20', '2026-07-22', 'Vacation', 'Pending']

list(reader) creates a two-dimensional list of rows and columns. leave_requests[1][0] returns Mia Johnson, but numeric positions become fragile when columns change.

The newline="" argument lets the csv module manage Windows line endings.

Create an Array of Dictionaries

For business data, I prefer csv.DictReader(). A dictionary stores values under named keys instead of numeric positions:

import csv

with open("leave_requests.csv", "r", newline="", encoding="utf-8") as file:
    reader = csv.DictReader(file)
    leave_requests = list(reader)

print(leave_requests[0])
print(leave_requests[0]["employee"])
print(leave_requests[0]["status"])

Output:

{'employee': 'Mia Johnson', 'start_date': '2026-07-20', 'end_date': '2026-07-22', 'leave_type': 'Vacation', 'status': 'Pending'}
Mia Johnson
Pending

Named fields improve readability because request["status"] communicates more than request[4]. The first CSV row supplies the keys.

You can filter the array with another list comprehension:

pending_requests = [
    request
    for request in leave_requests
    if request["status"].lower() == "pending"
]

print([request["employee"] for request in pending_requests])

Output:

['Mia Johnson', 'Olivia Brown']

Calling lower() handles Pending, PENDING, and pending. I also apply strip() because spreadsheet exports sometimes add invisible spaces.

Convert File Values to the Correct Data Type

Text files produce strings. Your application must convert those strings before performing calculations. Otherwise, Python may join numbers as text instead of adding them.

Suppose hours.txt contains:

8
7.5
4
8

Read the values as floating-point numbers:

with open("hours.txt", "r", encoding="utf-8") as file:
    hours = [float(line.strip()) for line in file if line.strip()]

print(hours)
print(sum(hours))

Output:

[8.0, 7.5, 4.0, 8.0]
27.5

The float type supports decimals. Use int() for whole numbers. Convert during import so later code receives consistent types.

Dates need the same treatment. The datetime module lets you convert a string to a date in Python for calendar calculations:

from datetime import datetime

request = {
    "start_date": "2026-07-20",
    "end_date": "2026-07-22"
}

start_date = datetime.strptime(
    request["start_date"], "%Y-%m-%d"
).date()

end_date = datetime.strptime(
    request["end_date"], "%Y-%m-%d"
).date()

duration = (end_date - start_date).days + 1

print(f"Calendar days requested: {duration}")

Output:

Calendar days requested: 3

strptime() parses text according to a format. %Y-%m-%d means year, month, and day. Adding one includes both dates.

This calculation counts weekends and U.S. holidays. Confirm the HR policy before calculating balances.

Read a JSON File into an Array in Python

JSON stores structured strings, numbers, booleans, arrays, and objects. It works well for files created by another application.

Add this content to leave_requests.json:

[
  {
    "employee": "Mia Johnson",
    "leave_type": "Vacation",
    "days": 3,
    "approved": false
  },
  {
    "employee": "Ethan Davis",
    "leave_type": "Jury Duty",
    "days": 1,
    "approved": true
  }
]

Read it with Python’s built-in json module. This module handles JSON data in Python without another package:

import json

with open("leave_requests.json", "r", encoding="utf-8") as file:
    leave_requests = json.load(file)

print(type(leave_requests))
print(leave_requests[0]["employee"])
print(leave_requests[0]["approved"])

Output:

<class 'list'>
Mia Johnson
False

json.load() reads JSON directly from an open file. The top-level JSON array becomes a Python list, each JSON object becomes a dictionary, and false becomes Python’s False.

Avoid using eval() to parse JSON or other file content. eval() can execute malicious Python code. The json module parses the supported data format without treating the file as a program.

Read Numeric File Data into a NumPy Array

A Python list handles basic numbers. A Python NumPy array stores numeric values efficiently and supports vectorization, which applies operations without manual loops.

Install NumPy from your terminal:

python -m pip install numpy

Then load hours.txt:

import numpy as np

hours = np.loadtxt("hours.txt", dtype=float)

print(hours)
print(type(hours))
print(hours.mean())

Output:

[8.  7.5 4.  8. ]
<class 'numpy.ndarray'>
6.875

The dtype=float argument requests decimal numbers. mean() calculates the average. NumPy fits data analysis, scientific computing, and large numeric datasets, but it adds an external dependency that simple text-processing scripts do not need.

If the numeric file includes a header or missing values, numpy.loadtxt() may fail. In that situation, NumPy genfromtxt offers options for headers and missing data:

import numpy as np

hours = np.genfromtxt(
    "hours_with_header.csv",
    delimiter=",",
    skip_header=1,
    usecols=1,
    dtype=float
)

print(hours)

Output: