Where can I find source or algorithm of Python's hash() function?

I don’t think the accepted answer is really representative of cPython’s internal hash implementations, which can be found in pyhash.c:

Description of hashing algorithm for numeric types:

/* For numeric types, the hash of a number x is based on the reduction
   of x modulo the prime P = 2**_PyHASH_BITS - 1.  It's designed so that
   hash(x) == hash(y) whenever x and y are numerically equal, even if
   x and y have different types.

   A quick summary of the hashing strategy:

   (1) First define the 'reduction of x modulo P' for any rational
   number x; this is a standard extension of the usual notion of
   reduction modulo P for integers.  If x == p/q (written in lowest
   terms), the reduction is interpreted as the reduction of p times
   the inverse of the reduction of q, all modulo P; if q is exactly
   divisible by P then define the reduction to be infinity.  So we've
   got a well-defined map

      reduce : { rational numbers } -> { 0, 1, 2, ..., P-1, infinity }.

   (2) Now for a rational number x, define hash(x) by:

      reduce(x)   if x >= 0
      -reduce(-x) if x < 0

   If the result of the reduction is infinity (this is impossible for
   integers, floats and Decimals) then use the predefined hash value
   _PyHASH_INF for x >= 0, or -_PyHASH_INF for x < 0, instead.
   _PyHASH_INF and -_PyHASH_INF are also used for the
   hashes of float and Decimal infinities.

   NaNs hash with a pointer hash.  Having distinct hash values prevents
   catastrophic pileups from distinct NaN instances which used to always
   have the same hash value but would compare unequal.

   A selling point for the above strategy is that it makes it possible
   to compute hashes of decimal and binary floating-point numbers
   efficiently, even if the exponent of the binary or decimal number
   is large.  The key point is that

      reduce(x * y) == reduce(x) * reduce(y) (modulo _PyHASH_MODULUS)

   provided that {reduce(x), reduce(y)} != {0, infinity}.  The reduction of a
   binary or decimal float is never infinity, since the denominator is a power
   of 2 (for binary) or a divisor of a power of 10 (for decimal).  So we have,
   for nonnegative x,

      reduce(x * 2**e) == reduce(x) * reduce(2**e) % _PyHASH_MODULUS

      reduce(x * 10**e) == reduce(x) * reduce(10**e) % _PyHASH_MODULUS

   and reduce(10**e) can be computed efficiently by the usual modular
   exponentiation algorithm.  For reduce(2**e) it's even better: since
   P is of the form 2**n-1, reduce(2**e) is 2**(e mod n), and multiplication
   by 2**(e mod n) modulo 2**n-1 just amounts to a rotation of bits.

   */

Hashing of doubles:

Py_hash_t
_Py_HashDouble(double v)
{
    int e, sign;
    double m;
    Py_uhash_t x, y;

    if (!Py_IS_FINITE(v)) {
        if (Py_IS_INFINITY(v))
            return v > 0 ? _PyHASH_INF : -_PyHASH_INF;
        else
            return _PyHASH_NAN;
    }

    m = frexp(v, &e);

    sign = 1;
    if (m < 0) {
        sign = -1;
        m = -m;
    }

    /* process 28 bits at a time;  this should work well both for binary
       and hexadecimal floating point. */
    x = 0;
    while (m) {
        x = ((x << 28) & _PyHASH_MODULUS) | x >> (_PyHASH_BITS - 28);
        m *= 268435456.0;  /* 2**28 */
        e -= 28;
        y = (Py_uhash_t)m;  /* pull out integer part */
        m -= y;
        x += y;
        if (x >= _PyHASH_MODULUS)
            x -= _PyHASH_MODULUS;
    }

    /* adjust for the exponent;  first reduce it modulo _PyHASH_BITS */
    e = e >= 0 ? e % _PyHASH_BITS : _PyHASH_BITS-1-((-1-e) % _PyHASH_BITS);
    x = ((x << e) & _PyHASH_MODULUS) | x >> (_PyHASH_BITS - e);

    x = x * sign;
    if (x == (Py_uhash_t)-1)
        x = (Py_uhash_t)-2;
    return (Py_hash_t)x;
}

Hashing of pointers:

Py_hash_t
_Py_HashPointerRaw(const void *p)
{
    size_t y = (size_t)p;
    /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid
       excessive hash collisions for dicts and sets */
    y = (y >> 4) | (y << (8 * SIZEOF_VOID_P - 4));
    return (Py_hash_t)y;
}

Py_hash_t
_Py_HashPointer(const void *p)
{
    Py_hash_t x = _Py_HashPointerRaw(p);
    if (x == -1) {
        x = -2;
    }
    return x;
}

Hashing of bytes for very short strings uses DJBX33A, otherwise uses the default hash:

Py_hash_t
_Py_HashBytes(const void *src, Py_ssize_t len)
{
    Py_hash_t x;
    /*
      We make the hash of the empty string be 0, rather than using
      (prefix ^ suffix), since this slightly obfuscates the hash secret
    */
    if (len == 0) {
        return 0;
    }

#ifdef Py_HASH_STATS
    hashstats[(len <= Py_HASH_STATS_MAX) ? len : 0]++;
#endif

#if Py_HASH_CUTOFF > 0
    if (len < Py_HASH_CUTOFF) {
        /* Optimize hashing of very small strings with inline DJBX33A. */
        Py_uhash_t hash;
        const unsigned char *p = src;
        hash = 5381; /* DJBX33A starts with 5381 */

        switch(len) {
            /* ((hash << 5) + hash) + *p == hash * 33 + *p */
            case 7: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
            case 6: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
            case 5: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
            case 4: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
            case 3: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
            case 2: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
            case 1: hash = ((hash << 5) + hash) + *p++; break;
            default:
                Py_UNREACHABLE();
        }
        hash ^= len;
        hash ^= (Py_uhash_t) _Py_HashSecret.djbx33a.suffix;
        x = (Py_hash_t)hash;
    }
    else
#endif /* Py_HASH_CUTOFF */
        x = PyHash_Func.hash(src, len);

    if (x == -1)
        return -2;
    return x;
}

The file also implements modified FNV hashing:

#if Py_HASH_ALGORITHM == Py_HASH_FNV
/* **************************************************************************
 * Modified Fowler-Noll-Vo (FNV) hash function
 */
static Py_hash_t
fnv(const void *src, Py_ssize_t len)
{
    const unsigned char *p = src;
    Py_uhash_t x;
    Py_ssize_t remainder, blocks;
    union {
        Py_uhash_t value;
        unsigned char bytes[SIZEOF_PY_UHASH_T];
    } block;

#ifdef Py_DEBUG
    assert(_Py_HashSecret_Initialized);
#endif
    remainder = len % SIZEOF_PY_UHASH_T;
    if (remainder == 0) {
        /* Process at least one block byte by byte to reduce hash collisions
         * for strings with common prefixes. */
        remainder = SIZEOF_PY_UHASH_T;
    }
    blocks = (len - remainder) / SIZEOF_PY_UHASH_T;

    x = (Py_uhash_t) _Py_HashSecret.fnv.prefix;
    x ^= (Py_uhash_t) *p << 7;
    while (blocks--) {
        PY_UHASH_CPY(block.bytes, p);
        x = (_PyHASH_MULTIPLIER * x) ^ block.value;
        p += SIZEOF_PY_UHASH_T;
    }
    /* add remainder */
    for (; remainder > 0; remainder--)
        x = (_PyHASH_MULTIPLIER * x) ^ (Py_uhash_t) *p++;
    x ^= (Py_uhash_t) len;
    x ^= (Py_uhash_t) _Py_HashSecret.fnv.suffix;
    if (x == -1) {
        x = -2;
    }
    return x;
}

static PyHash_FuncDef PyHash_Func = {fnv, "fnv", 8 * SIZEOF_PY_HASH_T,
                                     16 * SIZEOF_PY_HASH_T};

#endif /* Py_HASH_ALGORITHM == Py_HASH_FNV */

According to PEP 456, SipHash (MIT License) is the default string and bytes hash algorithm:

/* byte swap little endian to host endian
 * Endian conversion not only ensures that the hash function returns the same
 * value on all platforms. It is also required to for a good dispersion of
 * the hash values' least significant bits.
 */
#if PY_LITTLE_ENDIAN
#  define _le64toh(x) ((uint64_t)(x))
#elif defined(__APPLE__)
#  define _le64toh(x) OSSwapLittleToHostInt64(x)
#elif defined(HAVE_LETOH64)
#  define _le64toh(x) le64toh(x)
#else
#  define _le64toh(x) (((uint64_t)(x) << 56) | 
                      (((uint64_t)(x) << 40) & 0xff000000000000ULL) | 
                      (((uint64_t)(x) << 24) & 0xff0000000000ULL) | 
                      (((uint64_t)(x) << 8)  & 0xff00000000ULL) | 
                      (((uint64_t)(x) >> 8)  & 0xff000000ULL) | 
                      (((uint64_t)(x) >> 24) & 0xff0000ULL) | 
                      (((uint64_t)(x) >> 40) & 0xff00ULL) | 
                      ((uint64_t)(x)  >> 56))
#endif


#ifdef _MSC_VER
#  define ROTATE(x, b)  _rotl64(x, b)
#else
#  define ROTATE(x, b) (uint64_t)( ((x) << (b)) | ( (x) >> (64 - (b))) )
#endif

#define HALF_ROUND(a,b,c,d,s,t)         
    a += b; c += d;             
    b = ROTATE(b, s) ^ a;           
    d = ROTATE(d, t) ^ c;           
    a = ROTATE(a, 32);

#define DOUBLE_ROUND(v0,v1,v2,v3)       
    HALF_ROUND(v0,v1,v2,v3,13,16);      
    HALF_ROUND(v2,v1,v0,v3,17,21);      
    HALF_ROUND(v0,v1,v2,v3,13,16);      
    HALF_ROUND(v2,v1,v0,v3,17,21);


static uint64_t
siphash24(uint64_t k0, uint64_t k1, const void *src, Py_ssize_t src_sz) {
    uint64_t b = (uint64_t)src_sz << 56;
    const uint64_t *in = (uint64_t*)src;

    uint64_t v0 = k0 ^ 0x736f6d6570736575ULL;
    uint64_t v1 = k1 ^ 0x646f72616e646f6dULL;
    uint64_t v2 = k0 ^ 0x6c7967656e657261ULL;
    uint64_t v3 = k1 ^ 0x7465646279746573ULL;

    uint64_t t;
    uint8_t *pt;
    uint8_t *m;

    while (src_sz >= 8) {
        uint64_t mi = _le64toh(*in);
        in += 1;
        src_sz -= 8;
        v3 ^= mi;
        DOUBLE_ROUND(v0,v1,v2,v3);
        v0 ^= mi;
    }

    t = 0;
    pt = (uint8_t *)&t;
    m = (uint8_t *)in;
    switch (src_sz) {
        case 7: pt[6] = m[6]; /* fall through */
        case 6: pt[5] = m[5]; /* fall through */
        case 5: pt[4] = m[4]; /* fall through */
        case 4: memcpy(pt, m, sizeof(uint32_t)); break;
        case 3: pt[2] = m[2]; /* fall through */
        case 2: pt[1] = m[1]; /* fall through */
        case 1: pt[0] = m[0]; /* fall through */
    }
    b |= _le64toh(t);

    v3 ^= b;
    DOUBLE_ROUND(v0,v1,v2,v3);
    v0 ^= b;
    v2 ^= 0xff;
    DOUBLE_ROUND(v0,v1,v2,v3);
    DOUBLE_ROUND(v0,v1,v2,v3);

    /* modified */
    t = (v0 ^ v1) ^ (v2 ^ v3);
    return t;
}

static Py_hash_t
pysiphash(const void *src, Py_ssize_t src_sz) {
    return (Py_hash_t)siphash24(
        _le64toh(_Py_HashSecret.siphash.k0), _le64toh(_Py_HashSecret.siphash.k1),
        src, src_sz);
}

uint64_t
_Py_KeyedHash(uint64_t key, const void *src, Py_ssize_t src_sz)
{
    return siphash24(key, 0, src, src_sz);
}


#if Py_HASH_ALGORITHM == Py_HASH_SIPHASH24
static PyHash_FuncDef PyHash_Func = {pysiphash, "siphash24", 64, 128};
#endif

Objects such as tuples in (in tupleobject.c) have their own hashing methods. See the source for more examples:

static Py_hash_t
tuplehash(PyTupleObject *v)
{
    Py_uhash_t x;  /* Unsigned for defined overflow behavior. */
    Py_hash_t y;
    Py_ssize_t len = Py_SIZE(v);
    PyObject **p;
    Py_uhash_t mult = _PyHASH_MULTIPLIER;
    x = 0x345678UL;
    p = v->ob_item;
    while (--len >= 0) {
        y = PyObject_Hash(*p++);
        if (y == -1)
            return -1;
        x = (x ^ y) * mult;
        /* the cast might truncate len; that doesn't change hash stability */
        mult += (Py_hash_t)(82520UL + len + len);
    }
    x += 97531UL;
    if (x == (Py_uhash_t)-1)
        x = -2;
    return x;
}