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001 Ai Topological Sort

164-i-changed-my-python

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Python Internals - I Made Addition Unpredictable

Source: https://arpitbhayani.me/blogs/i-changed-my-python Date: 2020-01-03

Explore Python internals by tweaking its source code! Learn how I made addition unpredictable and the challenges I faced.

Did you ever take a peek at Python’s source code? I didn’t and hence I decided to have some fun with it this week. After cloning the repository I realized how well written is the code that makes python what it is. In the process of exploring the codebase, I thought of making some changes, not big optimizations but some minor tweaks that will help me understand how Python is implemented in C and along the course learn some internals. To make things fun and interesting I thought of changing how addition work by making it incorrect and unpredictable which means a + b will internally do one of the following operations, at random

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003 Rag Production
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004 Structure Of Llm Chat
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005 How Llms Work
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007 Defensive Databases
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011 Half Life
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012 Multi Paxos
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013 Mysql Replication Internals
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014 Bloom Filters
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016 Kafka Partitions
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017 Product Quantization
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018 Qkv Matrices
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019 Deleted Production
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020 How Llm Inference Works
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021 Blocking Queues
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022 Heartbeats In Distributed Systems
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029 Grpc Http2
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031 Growth Is Not About Doing Everything
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039 Masters
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041 Good Mentors Build People
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042 Always Have Back Burner Projects
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043 Before You Push Back Know What Youre Standing On
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044 Be The One They Can Count On
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045 How Much People Bet On You
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046 How To Get Leadership To Say Yes To Your Project
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047 Dont Let Your Best Ideas Die In Silence
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048 Be Someone Others Want To Work With
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049 Dont Fall For Xy Problem Ask Right Questions
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050 Biggest Lie Startups Tell Engineers
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051 Promotions Are Proactive Not Reactive
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052 Not Enough To Be Right Learn To Be Heard
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053 No One Ships Alone
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054 Not Every Mistake Needs A Correction
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055 Build Influence At Work
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056 Your Soft Skills Arent Soft At All
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057 Experience Before Forming Opinion
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058 Curiosity And High Bias For Action
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059 Worklog
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060 Mistakes And Growth
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061 Own It Instead Of Sweeping It Aside
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062 Dont Wait Step Up
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064 Interview Bias And What Sets You Apart
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072 We Engineers Suck At Task Estimation
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073 Shiny Object Syndrome In Tech
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074 3p
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075 Leverage The Equilibrium
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076 On Demand Container Loading In Aws Lambda
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077 Sql Has Problems We Can Fix Them Pipe Syntax In Sql
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078 Nanolog A Nanosecond Scale Logging System
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079 Best Resource Is Mythical
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080 Wtf The Who To Follow Service At Twitter
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085 Culture Fit
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086 Quantification In Resume
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087 Hiring Is Unfair
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088 Questions For Interviewers
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089 Collaboration Communication
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090 Out Of Vicious Interview Cycle
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091 Pitch Projects Not Ideas
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092 Read Design Docs
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093 Read Rca Docs
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094 Start Generalist
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095 Do Not Rely On Summaries
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096 Structure Your Design Interviews
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097 Title Inflation
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098 Find Your Own Project
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099 Six Pointers To Crack Coding And Design Interviews
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100 Keep Yourself Unblocked
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102 Pseudorandom Number Generation Lfsr
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105 Data Partitioning
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107 Conflict Resolution
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108 Conflict Detection
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110 Monotonic Reads
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112 Handling Outages Master Replica
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113 Replication Formats
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114 Replication Strategies
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115 Master Replica Replication
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116 Durability
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117 Isolation
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118 Atomicity
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119 Consistency
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120 Architectures In Distributed Systems
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121 Mistaken Beliefs Of Distributed Systems
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129 String Interning Python
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130 Recursion Visualizer Python
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132 2q Cache
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133 Israeli Queues
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134 1d Terrain
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135 Jaccard Minhash
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136 Ts Smoothing
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146 Inheritance C
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147 Rum
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148 Consistent Hashing
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160 Function Overloading
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161 Isolation Forest
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162 Image Steganography
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163 Long Integers Python
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164 I Changed My Python
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165 Benchmark And Compare Pagination Approach In Mongodb
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166 Mongodb Cursor Skip Is Slow
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168 Making Http Requests Using Netcat
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  • a + b
  • a - b
  • a * b
  • a / b
  • a ** b

After forking and cloning the source code of python, I broke down the task into following sub-tasks

  • find the entry point (the main function) of python
  • find where addition happens
  • find how to call other perform operations like subtraction, multiplication, etc on python objects.
  • write a function that picks one of the operators at random
  • write a function that applies an operator on the two operands

Before getting into how I did it, take a look below and see what it does

Random Math Operator in Python

You would see how performing addition on numbers 4 and 6 evaluates to 0, 10 and 24 depending on the operation it picked randomly.

Note, the change I made will only work when one of the operands is a variable. If the entire expression contains constants then it will be evaluated as regular infix expression.

Implementation

Operations in python work on opcodes very similar to the one that a microprocessor has. Depending on opcodes that the code is translated to, the operation is performed using operands (if required). The addition operation of python requires two operands and opcode is named BINARY_ADD and has value 23. When the executor encounters this opcode, it fetches the two operands from top of the stack, performs addition and then pushes back the result on the stack. The code snippet below will give you a good idea of what python does when it encounters BINARY_ADD.

Plain text

One thing to observe here is how it concatenates when both operands are unicode/string.

Checking if operands are numbers

For checking if both the operands for BINARY_ADD operation are numbers I used the predefined function named PyNumber_Check which checks if object referenced by PyObject is number or not.

Plain text
Writing a random function

For generating random integer I used the current time in seconds from the system using datetime.h library and took modulus with the max value. The code snippet below picks a random number from [0, max).

Plain text
Functions to perform other operations

Similar to the function PyNumber_Add which adds two python objects (if possible), there are functions named PyNumber_Subtract, PyNumber_Multiply, PyNumber_FloorDivide, and PyNumber_Power which performs operations as suggested by their names. I wrote a util function that takes two operands and an operator and returns the resulting python object after performing the required operation.

Plain text
The new BINARY_ADD implementation

Now as have everything required to make our BINARY_ADD unpredictable and following code snippet is very close to how it could be implemented.

Plain text

Challenges

After making all the required changes I ran make to build my new python binary and to my surprise, the code wouldn’t build. The reason was that the function where I made the changes was called during build and initialization phases and due to incorrectness induced in the BINARY_ADD the process ended in Segmentation Faults as now it has a function that instead of adding two numbers was subtracting, multiplying, dividing and raising to power at random.

To fix this issue I had to ensure that this random picking of operator only happened when the operation is asked from the interactive shell and should continue its normal execution for others. The function that gets called during an interactive shell is PyRun_InteractiveLoopFlags and hence I started passing a flag named source to all the functions till my trail reaches the opcode evaluation flow. The value of this source is set to 1 when it is triggered from the interactive shell for others the default value passed is 0. Once I had this source field in place with the proper value being passed from various initiations, everything worked like a charm.

You can find the detailed diff at github.com/arpitbbhayani/cpython/pull/1/files.

Conclusion

It was fun to change the python’s source code, I would recommend you to do this as well. It is always better if you know how things work internally and more importantly understand the complexities that are abstracted to make the application developers’ experience seamless.

You can find the source code at arpitbbhayani/cpython/tree/01-randomized-math-operators. Feel free to fork it and make some changes of your own and share it with me. I will be thrilled to learn what you did with it.

If you want to dive deep into python’s source I highly recommend you to read realpython.com/cpython-source-code-guide/. It is an excellent guide to get you started and understand the language semantics and coding practices of a core python developer. Once you know the basics, navigating through the codebase is a walk in the park.