Import statement behavior in Python - Step-by-Step Execution

Concept Flow - Import statement behavior

Start program

↓

Encounter import statement

↓

Check if module is loaded

↓

Use cached

↓

Access module contents

↓

Continue program execution

When Python sees an import, it checks if the module is already loaded. If yes, it uses the cached module. If not, it loads the module, then continues.

Execution Sample

Python

import math
print(math.sqrt(16))
import math
print(math.pi)

This code imports the math module twice and uses its functions and constants.

Execution Table

Step	Action	Module Loaded?	Result	Output
1	import math	No	Load math module
2	print(math.sqrt(16))	Yes	Call sqrt(16)	4.0
3	import math	Yes	Use cached math module
4	print(math.pi)	Yes	Access pi constant	3.141592653589793

💡 Program ends after printing results

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
math	undefined	module object	module object	module object	module object

Key Moments - 2 Insights

Why does importing the same module twice not reload it?

Because Python checks if the module is already loaded (see Step 3 in execution_table) and uses the cached version to save time.

What happens if you try to use a module before importing it?

You get an error because the module variable is undefined until the import runs (see variable_tracker start state).

Visual Quiz - 3 Questions

Look at the execution table, what is the output after Step 2?

A 4.0

B 16

C Error

D None

💡 Hint

Check the Output column at Step 2 in execution_table

At which step does Python decide to use the cached module instead of loading it again?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Module Loaded? and Result columns in execution_table

If the first import was removed, what would happen at Step 2?

A print would output 4.0 anyway

B Error because math is not defined

C math module would be loaded at Step 2

D Program would skip Step 2

💡 Hint

Refer to variable_tracker start state and key moment about using module before import

Concept Snapshot

import module_name

- Loads module if not loaded yet
- Uses cached module if already loaded
- Access module contents with module_name.member
- Importing twice does not reload
- Must import before use

Full Transcript

When Python runs a program and sees an import statement, it checks if the module is already loaded in memory. If not, it loads the module and makes it available. If yes, it uses the cached module to save time. This means importing the same module multiple times does not reload it again. You must import a module before using it, or you get an error. In the example, math is imported first, then used to calculate square root and access pi. The second import just uses the cached math module.

Practice

1. What happens when you use import module_name in Python?

easy

A. The module is copied into your current file.

B. Only the functions you call from the module are loaded.

C. The module code runs every time you call a function from it.

D. The entire module is loaded and its code runs once.

Solution

1. Step 1: Understand import behavior

   When you import a module, Python loads the whole module and runs its code once.

2. Step 2: Recognize module reuse

   After the first import, Python reuses the loaded module without running its code again.

3. Final Answer:

   The entire module is loaded and its code runs once. -> Option D

4. Quick Check:

   Import runs module once = A [OK]

Hint: Import runs module code once, then reuses it [OK]

Common Mistakes:

• Thinking module code runs every time a function is called
• Believing only used functions are loaded
• Assuming module code is copied into current file

2. Which of the following is the correct syntax to import only the sqrt function from the math module?

easy

A. from math import sqrt

B. import math.sqrt

C. import sqrt from math

D. from sqrt import math

Solution

1. Step 1: Recall import syntax for specific functions

   To import a specific function, use from module import function.

2. Step 2: Match syntax to options

   from math import sqrt matches this syntax: from math import sqrt.

3. Final Answer:

   from math import sqrt -> Option A

4. Quick Check:

   Specific import uses 'from module import item' = A [OK]

Hint: Use 'from module import name' to import parts [OK]

Common Mistakes:

• Using dot notation in import statement incorrectly
• Swapping module and function names
• Using 'import' with 'from' in wrong order

3. Consider two files:

# file1.py print('Loading file1') value = 10
# file2.py import file1 import file1 print(file1.value)

What is the output when running file2.py?

medium

A. Loading file1
Loading file1
10

B. Loading file1
10

C. 10
10

D. Error: module imported twice

Solution

1. Step 1: Understand module import behavior

   Python runs the module code only once, even if imported multiple times.

2. Step 2: Trace the output

   On first import, 'Loading file1' prints. Second import does nothing. Then prints value 10.

3. Final Answer:

   Loading file1 10 -> Option B

4. Quick Check:

   Module code runs once, reused later = B [OK]

Hint: Module code runs once, even if imported multiple times [OK]

Common Mistakes:

• Expecting module code to run twice
• Thinking repeated imports cause errors
• Confusing print output order

4. What is wrong with this code?

import math from math import sqrt print(math.sqrt(16))

medium

A. You cannot import the same module twice.

B. The code will cause a NameError.

C. There is no error; the code runs and prints 4.0.

D. You must use only one import style per module.

Solution

1. Step 1: Check import statements

   Importing the same module twice with different styles is allowed and does not cause error.

2. Step 2: Verify function call

   Calling math.sqrt(16) works and returns 4.0.

3. Final Answer:

   There is no error; the code runs and prints 4.0. -> Option C

4. Quick Check:

   Multiple imports allowed; function call works = D [OK]

Hint: Multiple imports of same module are allowed [OK]

Common Mistakes:

• Thinking multiple imports cause errors
• Confusing import styles must be exclusive
• Expecting NameError from this code

5. You have a module config.py with a variable setting = 5. In your main program, you do:

import config config.setting = 10 import config print(config.setting)

What will be printed and why?

hard

A. 10, because the module is loaded once and changes persist.

B. Error, because you cannot assign to module variables.

C. 5, because the second import reloads the module resetting variables.

D. None, because the variable is not accessible after import.

Solution

1. Step 1: Understand module import caching

   Python loads a module once and caches it; subsequent imports reuse the same module object.

2. Step 2: Analyze variable assignment and import

   Changing config.setting to 10 modifies the cached module. The second import does not reload, so the change remains.

3. Final Answer:

   10, because the module is loaded once and changes persist. -> Option A

4. Quick Check:

   Module cached; changes persist = C [OK]

Hint: Module imports cache; variable changes stay across imports [OK]

Common Mistakes:

• Assuming second import reloads module
• Thinking module variables are immutable
• Believing variable becomes inaccessible