sfs_py3.py

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High-level description

The code defines a class SFS that simulates the evolution of genetic sequences under a beta coalescent model and calculates the Site Frequency Spectrum (SFS) from the generated genealogical trees. The SFS represents the distribution of allele frequencies in a sample of DNA sequences.

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Code Structure

The SFS class inherits from the betatree class, which is responsible for generating the beta coalescent trees. The SFS class extends this functionality by accumulating allele frequency information from multiple trees to calculate and analyze the SFS.

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References

• betatree: The SFS class inherits from the betatree class, which is defined in the .betatree module.

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Symbols

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Symbol Name: SFS

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Description:

This class simulates the evolution of genetic sequences under a beta coalescent model and calculates the Site Frequency Spectrum (SFS).

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Inputs:

Name	Type	Description
sample_size	int	The number of individuals in the sample.
alpha	float	The alpha parameter of the beta coalescent model (default: 2).

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Outputs:

The class does not have a dedicated output method. Instead, it stores the calculated SFS in the self.sfs attribute.

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Internal Logic:

1. Tree Generation: The class utilizes the inherited coalesce() method from the betatree class to generate multiple beta coalescent trees.
2. Allele Frequency Calculation: For each tree, the code traverses the branches and records the number of descendants (weight) and branch length for each node. This information represents the allele frequency and time to coalescence, respectively.
3. SFS Calculation: The getSFS() method accumulates the branch lengths for each allele frequency across all generated trees, resulting in the SFS.
4. SFS Binning: The binSFS() method allows for binning the SFS using different modes (linear, log, logit) and bin numbers or custom bin edges.

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Side Effects:

• Modifies the self.alleles and self.sfs attributes during the SFS calculation.

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Symbol Name: glob_trees

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Description:

Generates multiple beta coalescent trees and stores the allele frequency information for each tree.

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Inputs:

Name	Type	Description
ntrees	int	The number of trees to generate (default: 10).

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Outputs:

The method does not return any value. It updates the self.alleles attribute with the allele frequency information from the generated trees.

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Internal Logic:

1. Iterative Tree Generation: The method iterates ntrees times, generating a new tree in each iteration using the coalesce() method.
2. Allele Information Extraction: For each tree, it extracts the weight (number of descendants) and branch length for all nodes (terminal and non-terminal) and appends this information to the self.alleles list.

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Symbol Name: getSFS

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Description:

Calculates the Site Frequency Spectrum (SFS) based on the generated trees.

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Inputs:

Name	Type	Description
ntrees	int	The number of trees to use for SFS calculation (default: 10).

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Outputs:

The method does not return any value. It updates the self.sfs attribute with the calculated SFS.

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Internal Logic:

1. Initialization: Creates a NumPy array self.sfs filled with zeros and a length equal to the sample size plus one.
2. SFS Accumulation: Iterates through the specified number of trees (ntrees) and for each tree:
   • Generates a new tree using the coalesce() method.
   • Traverses the tree nodes and increments the corresponding element in the self.sfs array based on the node’s weight (allele frequency) and branch length (time to coalescence).
3. Normalization: Divides the self.sfs array by the number of trees to obtain the average SFS.

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Symbol Name: binSFS

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Description:

Bins the pre-calculated SFS using different modes and bin numbers or custom bin edges.

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Inputs:

Name	Type	Description
mode	str	The binning mode. Options: “logit” (default), “linear”, “log”.
bins	int or array-like	The number of bins or an array-like object specifying the bin edges.

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Outputs:

The method does not return any value. It updates the self.binned_sfs, self.bin_center, and self.bin_width attributes based on the chosen binning parameters.

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Internal Logic:

1. Bin Edge Determination: Determines the bin edges based on the chosen mode and bins parameters.
   • If bins is an iterable, it is used directly as bin edges.
   • Otherwise, bin edges are calculated based on the chosen mode:
     • “logit”: Logit-spaced bins.
     • “linear”: Linearly spaced bins.
     • “log”: Logarithmically spaced bins.
2. SFS Binning: Uses the np.histogram() function to bin the pre-calculated SFS (self.sfs) based on the determined bin edges.
3. Normalization: Divides the binned SFS by the bin widths to obtain the density.

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Symbol Name: saveSFS

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Description:

Saves the calculated SFS to a file.

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Inputs:

Name	Type	Description
fname	str	The filename to save the SFS to. If the filename ends with “.gz”, the file will be gzipped.

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Outputs:

The method does not return any value. It saves the SFS to the specified file.

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Internal Logic:

1. SFS Calculation (if necessary): If the SFS has not been calculated yet (self.sfs is None), it calls the getSFS() method to calculate it.
2. File Saving: Uses the np.savetxt() function to save the SFS to the specified file.

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Symbol Name: loadSFS

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Description:

Loads a previously saved SFS from a file.

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Inputs:

Name	Type	Description
fname	str	The filename to load the SFS from.
alpha	float	The alpha parameter of the beta coalescent model (optional).

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Outputs:

The method does not return any value. It updates the self.n, self.sfs, and self.alpha attributes based on the loaded SFS.

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Internal Logic:

1. File Loading: Attempts to load the SFS from the specified file using the np.loadtxt() function.
2. Data Validation: Checks if the loaded data is a one-dimensional vector. If not, it prints an error message and sets self.sfs to None.
3. Attribute Update: If the loaded data is valid, it updates the self.n (sample size), self.sfs, and self.alpha attributes accordingly.

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Dependencies

Dependency	Purpose
numpy	Numerical computations, array operations, and file I/O.
scipy.special	Special functions, including the gamma function used in the beta distribution.
Bio.Phylo	Phylogenetic tree manipulation and analysis.

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Error Handling

The code includes basic error handling for file loading in the loadSFS method. If the specified file does not exist or the loaded data is not a one-dimensional vector, it prints an error message and sets the self.sfs attribute to None.

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Logging

The code does not implement any specific logging mechanisms.

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API/Interface Reference

The code does not explicitly define an API or public interface.